Bud rot is one of the most devastating things a grower can deal with. This day and age, when a positive microbial test can cause a grower to lose their crop, there is zero tolerance for fungal pathogens. Bud rot can be especially devastating indoors. If there are conducive conditions and susceptible varieties in close proximity, spores can quickly and easily spread throughout the entire grow groom.

What causes bud rot?

Bud rot is cause by Botrytis cinerea. which is the name for the anamorph (asexual form) of this fungus; sexual reproduction is rare in this species. It is an aggressive necrotrophic plant pathogen that causes disease on over 1,000 crops [1]. It is responsible for up to $100 billion in annual crop losses worldwide [2]. B. cinerea can infect a wide range of tissue types including fruits, flowers, leaves, stems, and storage tissues. It is a serious issue for both preharvest and postharvest (i.e. that characteristic fuzzy gray mold that grows on your strawberries that you get from the supermarket). However, sometimes this fungus can be desirable. Some grape growers utilize the fungus on wine grapes to produce ‘noble rot’ wines. These high value grapes have enhanced ripening responses and may have greater levels of sugars and flavor compounds. In Cannabis, bud rot is never desirable.

Luckily for us, B. cinerea is one of the best studies fungal organisms in the history of plant pathology. It is often used as the model organism for studying necrotrophic plant pathogens. B. cinerea is not just a mold that grows passively on your buds like Penicillium does on bread, it employs mechanisms to both suppress plant defenses and kill plant tissues.

Powdery Mildew and Bud Rot are Both Labeled as ‘Mold’. How do they Differ?

Let us compare B. cinerea to one of the other most damaging fungal disease of Cannabis: powdery mildew (PM). PM is a biotrophic fungus, meaning that it is an obligate parasite that requires living cells to extract nutrients from. Because of this, PM species are required to evolve strategies to specifically overcome the defenses of particular plant species and manipulate plant immune responses (this leads to relatively narrow host ranges for any given species of PM fungi). B. cinerea on the other hand, is much less precise in its pathogenicity strategies.

It tends to rely much more heavily on the secretion of plant toxins and enzymes to kill and digest plant tissues as opposed to suppressing cellular immune responses and maintaining host viability. The fungus can then feed on the dead tissue without having to defend itself against further attack by the plants. This partly explains the much wider host range of B. cinerea, as many of the toxins and cell wall degrading enzymes will ubiquitously kill a wide variety of plant tissues from a wide variety of plant species. As a rule of thumb, biotrophic plant pathogens rely most heavily on secreted effector proteins that manipulate the host plant’s defense response, while necrotrophic plant pathogens rely most heavily on phytotoxins, cell wall degrading enzymes, and other extracellular enzymes.

How does B. cinerea infect plants?

Though B. cinerea has long been considered a necrotroph, newer literature is beginning to show that B. cinerea might be better classified as a hemibiotroph, meaning that it begins its life as a parasite and later transitions to a necrotrophic lifestyle . In order to succeed as a necrotroph, a fungus first has to gain a foothold in the plant, and it does this by suppressing plant immune responses for a short period of time until enough fungal biomass has accumulated in the host plant to successfully switch to a necrotrophic lifestyle and kill surrounding plant cells. For Botrytis, this is not a long time, and the very first symptoms of infection by Botrytis are necrotic lesions that begin spreading, showing that the parasitic phase is quite short-lived indeed.

So how do we know that there is a parasitic phase? As described earlier, necrotrophs must kill plant cells to feed on. However, in initial infection events, Botrytis actually interferes with the plant’s programmed cell death immune function known as the hypersensitive response (HR). The hypersensitive response is a common way for plants to defend against pathogen invasion by killing cells under attack and bolstering the defenses of neighboring cells. One might think that this is beneficial to necrotrophs, but it appears that in order to initially establish itself in the host, Botrytis must suppress PCD by secreting effector proteins and small RNAs that interfere with normal plant responses [3]. Secreted small RNAs from Botrytis ‘hijack’ the RNA interference system in plants and help silence genes involved in plant immunity [4]. Another factor that challenges the notion of Botrytis as an obligate necrotroph is the fact that it can sometimes colonize plants asymptomatically, although I am not aware of this being demonstrated yet in Cannabis [5]. This raises the question of whether B. cinerea is a fairly ubiquitous endophyte that only causes serious disease when both the environment and host are conducive to disease development (there may be cases where a necrotrophic phase is not tiggered).

What is going on biochemically during plant infection?

After accumulating biomass within the host, Botrytis can switch to a necrotrophic lifecycle, and it employs a variety of pathogenicity tactics, including inducing the HR response in plant cells rather than suppressing it. For instance, it begins to produce macromolecular toxins that can induce plant cell death [6, 7]. One such metabolite, oxalic acid, may induce these responses through acidification of plant tissues, leading to production and activation of various fungal enzymes including pectinases (pectin degrading enzymes), laccases (lignin-degrading enzymes), and proteases (protein degrading enzymes) [8, 10]. Furthermore, oxalic acid can weaken cell walls by chelating calcium ions, which is necessary for forming calcium pectate in plant cell walls [10]. In a closely related necrotrophic species, Sclerotinia sclerotiorum, oxalic acid has been shown to trigger host HR response [9]. Botrytis has also been found to produce plant hormones and/or induce plant hormone changes including ethylene and abscisic acid levels, both of which have been found to make plants more susceptible to Botrytis infection [11, 12, 13, 14]. Furthermore, different effector proteins are expressed at different points of infection. BcSpl1 is one such protein that is more abundant in later infection stages and is an important virulence factor for B. cinerea, inducing cell death [15]. Another effector, BcIEB1, may help protect the fungus from a host-produced antifungal, osmotin [16]. Most putative effector genes in Botrytis are not well characterized, and some effectors may not contribute to virulence due to host plants evolving ways to recognize the effectors and trigger defense responses, such as is the case with BcNEP1 and BcNEP2 [17].

What is the life cycle of Botrytis?

As previously mentioned, Botrytis usually functions as an asexual fungus. In the spring, fungal mycelium and sclerotia (defined later) begin to grow/germinate and make conidiophores that produce asexual spores known as conidia. Spores are usually dispersed through wind and water. In the presence of moisture (unlike PM, Botrytis spores require free water to germinate), the conidia (asexual spores) germinate and produce structures known as appressoria to penetrate plant cuticles through immense pressure buildup. The invasive hyphae can then grow within the extracellular space of plant tissues.

As previously discussed, they initially suppress plant responses and attempt to remain undetected while accumulating biomass. After a short period of growth, the fungus switches to a necrotrophic phase and begins to kill and rot proximal cells. After this initial foothold is established, the hyphae can continue to invade the host tissues and the fungus can produce more conidiophores on the plant tissue surface. This acts a secondary inoculum source that amplifies the spread of the disease within a given season (polycylclic disease). Come winter, the fungus begins to produce structures called sclerotia that are dense survival structures composed of highly melanized mycelium. Furthermore, mycelium can survive within dead plant tissue through the winter and produce conidia the following spring. The sexual phase is rare, but occurs when the sclerotia produce a fruiting body known as an ascocarp (analagous to a mushroom) that releases sexual spores known as ascospores.

What does bud rot look like in Cannabis? How do I diagnose it?

This disease displays both symptoms (visible effects on plant tissues) as well as signs (seeing the actual fungal tissue). However, long before you see a tuft of gray mold coming out of your buds, you will notice flagging of plant colas. This means that the tops of your colas will begin to dieback, leaving brown tissue as shown below:

You might also first notice symptoms on foliage. If your plant is not showing signs of nutrient burn or natural senescence of late flower, it can be easy to spot random leaves that have turned brown and dry, such as below:

The picture above shows signs of visible mycelium within the bud. However, if bud rot continues to progress, aerial mycelium will begin to grow as tufts out of your buds such as this:

I personlly lost a plant to bud rot when I first started growing because I attempted to grow in an unvented tent placed in a closet indoors without a humidifier, so I know how devastating it can be to see something like this on what you have worked so hard to produce.

How Do I Prevent Getting Bud Rot?

As with all plant diseases, it is important to keep in mind the disease triangle, which states that for a disease to develop, there needs to be a susceptible host, a conducive environment, and a virulent pathogen.

Virulent Pathogen

Unless you are in a perfectly enclosed environment, you have to assume that Botrytis spores are fairly ubiquitous in the environment. If you are within a contained grow, you have a bit of control over this factor, and you can do a few things to try to keep potential inoculum levels low.

• UV lamps: Using supplemental UV lighting is recommended for increasing secondary metabolite production in plants (for more info on this, check out my post on grow lights under the Home Growing Made Easy page). It also has the added benefit of helping reduce spore inoculum levels. Beyond your supplemental UV lights, you can add UV-B lamps within the ducting in your sealed room to help sanitize the air.
• HEPA filters: Using HEPA filters in your grow room will help remove spores from the air through active filtration.
• Ozone: Ozone generators may be helpful in reducing the growth of Botrytis. One study found reduced growth rates in air with 1.5 uL/L of ozone [21]. However, ozone may have human health risks and may have negative effects on your plants at high concentrations.
• Use dedicated clothing for your grow space can help prevent you from bringing in spores from the clothes you wear out in public. If you really want to take an extra step, you can invest in some Painters’ coveralls.
• Have good cultural practices including sanitizing hands, tools, shoes etc. regularly. Always sterilize your tools frequently and sterilize your entire grow area after each harvest using techniques such as burning sulfur, spraying with 10% bleach, or Quat soaps. If you have infected tissues, do not mulch it into your soil at the end of the grow. cut off infected buds as soon as possible.

Susceptible Host

Most people do not select their plants based on their resistance to bud rot, though it can certainly be a factor. Growers (particularly indoor growers) generally choose what they are growing based on the market demand of what consumers want to smoke, and they rely on other practices to try to prevent their crop from getting bud rot. However, despite the lack of studies of this disease in Cannabis, one can look to community forums for discussion of strains that may show promise of being relatively resistant to bud rot.

In general, it is useful to consider what environments favor bud rot development. Landrace strains from regions of the world that have favorable conditions for bud rot likely experience the most pressure from Botrytis and are the most likely to have evolved resistance to the pathogen. Based on this, it is likely that Cannabis landrace strains from equatorial, moderately warm, humid, and wet regions of the world have developed greater resistance to bud rot than landrace strains from cold, drier climates. This is basically opposite of strains with higher levels of PM resistance. For instance, strains with Afghani heritage generally are more PM resistant than many equitorial sativa strains, whereas these equitorial strains generally are more resistant to bud rot than Afghani Cannabis plants.

One reason that I would recommend sativa strains that evolved in wet, moderate-warm climates is obvious from their plant structure: sativa buds are not as dense, internodal spacing is greater than indica plants, leaves are much thinner, and in general, the structure of these plants is conducive to high amounts of airflow and avoids wet microclimates within dense buds. Beyond the macro structure of the plant, there are likely more complex molecular interactions going on that contribute to a genotype’s resistance to bud rot.

It certainly is not a guarantee that growing a particular strain ensures bud rot resistance. It is also important to know that there can be genetic variability within a strain when it comes to disease resistance, and a bad enough environment may still be able to overcome resistant strains.

Examples of landrace strains with putative resistance to bud rot include:

1. Durban Poison is a 100% inbred sativa plant from a wam, wet region of South Africa
2. Brazil Amazonia is a landrace strain from one of the most conducive environments in the world to bud rot- the Amazon Jungle. However, it somewhat breaks the stereotype of what plants do best in these climates: it has an indica-like phenotype with denser buds and quicker flowering times than other strains on this list.
3. Colombian Gold- Another landrace sativa from the tropics. Colombia is one of the most ecologically diverse regions in the world.
4. Thai Sativa- from one of the most conducive regions in the world to bud rot, the Thai sativa is a very long-flowering strain with a classic sativa phenotype: lanky, thin leaves, large internodal spacing, buds not particularly dense.

These particular genetics are not really viable in a modern Cannabis market, aside from the occasional old-school stoner or a connoisseur of unique phenotypes. They are generally difficult to grow, low-yielding, and low-potency. However, there are many modern hybrid strains that have been bred from these landrace genetics.

Modern, commercially viable sativa strains:

• Ghost Train Haze
• Bruce Banner
• Chernobyl
• Amnesia Haze
• Hawaiian Maui Wowie
• Sour Diesel
• Laughing Buddha
• Bay 11
• G13 Haze
• Trainwreck
• Casey Jones
• Super Lemon Haze
• Grapefruit
• Red Dragon
• Cannalope Haze
• Destroyer
• Jamaican Dream
• Pineapple Express
• AK-47
• Hulkberry
• Jack Herer
• Chemdawg

Again, I am not making the claim that you will not get bud rot if you grow with these strains, but it certainly can make a difference to pick a strain with genetic history of sativa landrace from wet climates. Strain selection is most important for those growing outdoors in a moderate temperature, wet climate. If you have a dry environment or can control your environment, it should be possible to prevent bud rot with proper defoliation, training, and grow area design/IPM)

Conducive Environment

As mentioned previously, B. cinerea thrives in moderate temperature, humid, and wet environments. If moisture from dew or rain accumulates on plant buds, it is likely that there is a microclimate that develops in your buds with up to 100% relative humidity and free water that can stimulate Botrytis conidia germination. While Botrytis can infect foliage and stems, causing lesions, it is far less common than flower infections. This is likely due to the conducive microclimate in buds as compared to the microclimates surrounding leaves and stems, though floral tissues may also have less defensive abilities than these other tissue types.

There are a few basic tips to help prevent bud rot.

1. Have high amounts of air circulation in your growing area. Not only will it help with drying out wet tissues quickly, it will help disrupt microclimates that may form where there is stagnant air, reducing local spikes in humidity and temperature. Have a good amount of fans, and do proper training and defoliation in order to maintain that breezes can reach every part of your plant.
2. In my opinion, temperature can be important, but it is less important than controlling humidity, moisture and airflow. For instance, if you are in flowering stage and are supplementing CO2 in a commercial grow, it is important to maximize your yield and to make the most of high light and CO2 levels, a temperature around 80F is ideal for plant growth while maintaining most of the monoterpenes in your buds. This temperature is conducive to bud rot, but a lower temperature may be even more conducive. It seems that for Botrytis, moderate temperatures around 60-75F are most conducive for plant infection [18]. However, in greenhouse tomatoes, it was found that flower infections increased at higher temperatures (77F was the highest used in the study), whereas stem infections decreased. Based on this, it may be that Botrytis can thrive in Cannabis flowers at higher temperatures. I feel comfortable recommending that grow areas be kept at 77-82F during flower for the sake of plant evapotranspiration and growth, and I would not worry too much about temperatures dropping at night or reducing day temperatures in late flower to induce color changes and/or preserve some monoterpenes.
3. Humidity: For those familiar with VPD (vapor pressure deficit), it is a figure that indicates the rate of evapotranspiration from plants dependent upon leaf temperature and local humidity. It is a good guide for grow room environmental settings to maximize plant growth, and a chart showing the ideal humidity level for different leaf temperatures is shown below:

After switching to flower, at 27C (around 80F), VPD recommendations would be to keep humidity around 55% until late flower, when it can be dropped to around 42%. For grapes, these recommendations should be sufficient to keep B. cinerea from taking hold. 65% RH or lower seems to be sufficient to prevent Botrytis in grape [25]. In a study done one rose petals, an RH of 92% at 15C was required to cause lesions within 24 hours [19]. However, it is imporant to keep in mid that Cannabis buds have a unique structure that can have very humid microclimates in the buds compared to these other plants.

In tomato, mild disease symptoms were found at humidity as low as 56% RH. Unlike in tomato, Cannabis should have a zero tolerance policy for bud rot. When inhaling spores, certain immunosuppressed individuals can actually develop infections from inhaling Botrytis spores. Furthermore, the microclimates formed within Cannabis buds can significantly raise the humidity of the air in the flowers above that found in the grow space. While it is not ideal on the VPD chart, I recommend keeping humidity at 50% RH during early flower, and 40% RH during the last few weeks of flower. It is not worth trying to gain a marginal yield increase while risking your entire crop to bud rot. I believe the quality/yield is still maintained at this RH, but if you would like to keep it higher, 50% RH is likely okay. I tend to recommend staying on the safe side when it comes to avoiding mold issues altogether.

Application-Based Control Methods

Given that there are few approved fungicides with a targeted mode of action, my recommendations will mostly be based on approved fungicides in California.

Plant oil

I am not a big fan of using neem oil in mid-late flower, mostly because of certain health risks associated with neem. However, I certainly believe in a good regimen of spraying neem in vegetative growth and early flower. In addition to spraying about every 10 days during vegetative growth, I recommend doing one application when you flip your light cycles to 12/12 and doing another application at around 2 weeks of flower. I would not apply neem after this point. However, one could continue using other oils including triglycerides such as cottonseed oil or soybean oil, or even a product such as Trifecta crop control that utilizes corn oil and essential oils from plants such as garlic, thyme, peppermint, and rosemary. It also contains citric acid which may help control fungi.

Personally, I prefer to stop using all oils about 2-3 weeks into flower depending on the strain, and I begin alternating a biofungicide such as Stargus or Serenade with a potassium bicarbonate product such as Green Cure. Every 5 days I spray one of the products, alternating between the two. However, for the last 2-3 weeks, I also stop applying the biofungicide and only spray potassium bicarbonate once per week excluding the last week.

Biofungicides

I believe that biofungicides are good to use for prevention of bud rot. They are not toxic and can be sprayed up to the day of harvest. We focus on soil health and the microbial community in plants’ rhizospheres, but we tend to not pay as much attention to the phylosphere of plants. In regards to premade products, I recommend spraying Bacillus amyloliquefaciens products such as Double Nickel 55 or Stargus during flower. I only say this because it is the only approved Bacillus spray to use in California, but Bacillus subtilis sprays such as Serenade are good. I would do the first spray at first sign of flower and reapply every 10 days for 3-5 applications. Essentially, this bacteria will be able to colonize the aerial portions of the plant and help prevent initial colonization from Botrytis.

pH Agents

I believe that all Cannabis growers should have a good spray schedule of either a citric acid-based product (i.e. Nuke Em by Flying Skull or Plant Therapy by Lost Coast) or a potassium bicarbonate product (i.e. Green Cure). These products help inhibit fungal spore germination by altering the surface pH of plant tissues. I would not use both products, since citric acid works by acidifying the surface while bicarbonate works by basifying the surface. Both citric acid and potassium bicarbonate have been found to inhibit Botrytis spore germination [22]. There is no research in Cannabis comparing these products, but both appear to be inhibitory in vitro. However, we also know that Botrytis functions partly by acidifying the plant tissues. We also have some evidence in postharvest rot of kiwi fruit, that citric acid may make disease incidence worse, while potassium bicarbonate is effective for control [23, 24]. For these reasons, I recommend using a potassium bicarbonate spray such as green cure weekly during flower, even up to the day of harvest.

1. Fillinger, S., & Elad, Y. (2016). Botrytis: the fungus, the pathogen and its management in agricultural systems. Springer.
2. Hua, L., Yong, C., Zhanquan, Z., Boqiang, L., Guozheng, Q., & Shiping, T. (2018). Pathogenic mechanisms and control strategies of Botrytis cinerea causing post-harvest decay in fruits and vegetables. Food Quality and Safety, 2(3), 111–119. https://doi.org/10.1093/fqsafe/fyy016
3. Veloso, J., & van Kan, J. A. L. (2018). Many shades of grey in Botrytis–host plant interactions. Trends in Plant Science, 23(7), 613–622.
4. Wang, M., Weiberg, A., Dellota Jr, E., Yamane, D., & Jin, H. (2017). Botrytis small RNA Bc-siR37 suppresses plant defense genes by cross-kingdom RNAi. RNA Biology, 14(4), 421–428.
5. Ngah, N., Thomas, R. L., Shaw, M. W., & Fellowes, M. D. E. (2018). Asymptomatic Host Plant Infection by the Widespread Pathogen Botrytis cinerea Alters the Life Histories, Behaviors, and Interactions of an Aphid and Its Natural Enemies. Insects, 9(3), 80. https://doi.org/10.3390/insects9030080
6. Huo, D., Wu, J., Kong, Q., Zhang, G. B., Wang, Y. Y., & Yang, H. Y. (2018). Macromolecular Toxins Secreted by Botrytis cinerea Induce Programmed Cell Death in Arabidopsis Leaves. Russian Journal of Plant Physiology, 65(4), 579–587. https://doi.org/10.1134/S1021443718040131
7. Govrin, E. M., Rachmilevitch, S., Tiwari, B. S., Solomon, M., & Levine, A. (2006). An elicitor from Botrytis cinerea induces the hypersensitive response in Arabidopsis thaliana and other plants and promotes the gray mold disease. Phytopathology, 96(3), 299–307.
8. Manteau, S., Abouna, S., Lambert, B., & Legendre, L. (2003). Differential regulation by ambient pH of putative virulence factor secretion by the phytopathogenic fungus Botrytis cinerea. FEMS Microbiology Ecology, 43(3), 359–366.
9. Kim, K. S., Min, J.-Y., & Dickman, M. B. (2008). Oxalic acid is an elicitor of plant programmed cell death during Sclerotinia sclerotiorum disease development. Molecular Plant-Microbe Interactions : MPMI, 21(5), 605–612. https://doi.org/10.1094/MPMI-21-5-0605
10. Petrasch, S., Knapp, S. J., van Kan, J. A. L., & Blanco-Ulate, B. (2019). Grey mould of strawberry, a devastating disease caused by the ubiquitous necrotrophic fungal pathogen Botrytis cinerea. Molecular Plant Pathology, 20(6), 877–892. https://doi.org/10.1111/mpp.12794
11. Takino, J., Kozaki, T., Ozaki, T., Liu, C., Minami, A., & Oikawa, H. (2019). Elucidation of biosynthetic pathway of a plant hormone abscisic acid in phytopathogenic fungi. Bioscience, Biotechnology, and Biochemistry, 83(9), 1642–1649. https://doi.org/10.1080/09168451.2019.1618700
12. Blanco-Ulate, B., Vincenti, E., Powell, A. L. T., & Cantu, D. (2013). Tomato transcriptome and mutant analyses suggest a role for plant stress hormones in the interaction between fruit and Botrytis cinerea. Frontiers in Plant Science, 4, 142.
13. Valero-Jiménez, C. A., Veloso, J., Staats, M., & van Kan, J. A. L. (2019). Comparative genomics of plant pathogenic Botrytis species with distinct host specificity. BMC Genomics, 20(1), 203. https://doi.org/10.1186/s12864-019-5580-x
14. Chague, V., Elad, Y., Barakat, R., Tudzynski, P., & Sharon, A. (2002). Ethylene biosynthesis in Botrytis cinerea. FEMS Microbiology Ecology, 40(2), 143–149. https://doi.org/10.1111/j.1574-6941.2002.tb00946.x
15. Frías, M., González, C., & Brito, N. (2011). BcSpl1, a cerato-platanin family protein, contributes to Botrytis cinerea virulence and elicits the hypersensitive response in the host. New Phytologist, 192(2), 483–495. https://doi.org/10.1111/j.1469-8137.2011.03802.x
16. González, M., Brito, N., & González, C. (2017). The Botrytis cinerea elicitor protein BcIEB1 interacts with the tobacco PR5-family protein osmotin and protects the fungus against its antifungal activity. New Phytologist, 215(1), 397–410. https://doi.org/10.1111/nph.14588
17. Arenas, Y., Kalkman, E., Schouten, A., Vredenbregt, P., Dieho, M., Uwumukiza, B., & Kan. (2007). Functional analysis of Botrytis cinerea nep-like proteins.
18. Greenhouse & Floriculture: Botrytis Blight of Greenhouse Crops | UMass Center for Agriculture, Food and the Environment. (n.d.). Retrieved March 5, 2020, from https://ag.umass.edu/greenhouse-floriculture/fact-sheets/botrytis-blight-of-greenhouse-crops
19. Williamson, B., Duncan, G. H., Harrison, J. G., Harding, L. A., Elad, Y., & Zimand, G. (1995). Effect of humidity on infection of rose petals by dry-inoculated conidia of Botrytis cinerea. Mycological Research, 99(11), 1303–1310. https://doi.org/https://doi.org/10.1016/S0953-7562(09)81212-4
20. EDEN, M. A., HILL, R. A., BERESFORD, R., & STEWART, A. (1996). The influence of inoculum concentration, relative humidity, and temperature on infection of greenhouse tomatoes by Botrytis cinerea. Plant Pathology, 45(4), 795–806. https://doi.org/10.1046/j.1365-3059.1996.d01-163.x
21. Nadas, A., Olmo, M., & García, J. M. (2003). Growth of Botrytis cinerea and Strawberry Quality in Ozone-enriched Atmospheres. Journal of Food Science, 68(5), 1798–1802. https://doi.org/10.1111/j.1365-2621.2003.tb12332.x
22. Fayza Tahiri Alaoui, Latifa Askarne, Hassan Boubaker, El Hassane Boudyach and Abdellah Ait Ben Aoumar, 2017. Control of Gray Mold Disease of Tomato by Postharvest Application of Organic Acids and Salts. Plant Pathology Journal, 16: 62-72.
23. Pennycook, S. R. (1986). Citric acid dipping of kiwifruits promotes Botrytis storage rot. New Zealand Journal of Experimental Agriculture, 14(2), 205–207. https://doi.org/10.1080/03015521.1986.10426144
24. Turkkan, M., Özcan, M., & Erper, İ. (2017). Antifungal effect of carbonate and bicarbonate salts against Botrytis cinerea, the casual agent of grey mould of kiwifruit. Akademik Ziraat Dergisi, 6, 107–114. https://doi.org/10.29278/azd.371066
25. Ciliberti, N., Fermaud, M., Roudet, J., & Rossi, V. (2015). Environmental Conditions Affect Botrytis cinerea Infection of Mature Grape Berries More Than the Strain or Transposon Genotype. Phytopathology, 105(8), 1090–1096. https://doi.org/10.1094/PHYTO-10-14-0264-R

I am not sponsored by any companies that make the products I recommend. However, as an Amazon associate, I earn from qualifying purchases made through provided links.

I also recommend checking out Growbuds USA for a wide selection of high-quality LED grow lights at a bargain.

First off, let me say that there is not grow light on the market that will compete with the cost efficiency and the spectrum of sunlight. Of course, there is some argument as to whether indoor grown Cannabis is a higher quality than outdoor Cannabis, which in some cases, may be true. I believe in these cases, it is not the spectrum of the sunlight that is to blame, it is usually other environmental factors. If you are lucky enough to grow outdoors or have access to greenhouse growing in a good environment, I recommend you take full advantage of that. The purpose of this article is to educate you on indoor lighting and supplemental greenhouse lighting for Cannabis, it is not advocating that growing under synthetic light is the only proper way to do it.

There are a wide range of lighting options available to purchase including light emitting diode (LED) lights, high intensity discharge (HID) lights such as high pressure sodium (HPS), metal halide (MH), ceramic metal halide (CMH) lights, or fluorescent lights such as T5 lights. A beginning grower can be quite overwhelmed by the options, and many would just appreciate a recommendation of what kind of light to buy for their purposes. In this article, I will attempt to educate you on the pros and cons of each type of light and provide some recommendations of lights for different purposes and price points.

Just a disclaimer: I have only ever grown using MH/HPS lights. For me, the reason for this is simply the low up-front cost and an okay level of efficiency. If I had more money to spend, I would likely upgrade to a CMH bulb with supplemental red light and UV bulbs or a full spectrum high-quality LED with supplemental UV light.

Terms you need to know

Color Warmth– This term is only relevant for broad spectrum grow lights that have red, green, and blue light represented in the spectrum; ‘blurple’ (red and blue diodes) grow lights are not really represented in this measurement. While it is more useful to actually see the wavelength spectrum of your grow light, the color warmth measurement can give you an idea of what the spectrum is like. Color warmth is a description of how red or blue your eyes perceive the light emitted from your grow light. It is denoted by a number followed by the letter ‘K’, and is on a scale from 1,000K to 10,000K. The ‘K’ stands for Kelvin, but is not related to the unit of temperature.

• 2000K-3000K: Warm white, illumination appears yellowish-orangish (the spectrum has relatively more red light than other wavelengths; this is the range most broad spectrum grow lights fall in)
• 3100K-4500K: Cool white, illumination appears fairly neutral
• 4600K-6500K: Appears more similar to daylight and is more blueish than the previous two.

PPFD (Photosynthetic Photon Flux Density)

This is a spot measurement of how many photons are falling on a particular area withing a given time. It is measured in units of micromoles of photons per square meter per second. Though the units are in square meters, the measurement is usually taken with a small light meter, and so it is best to take multiple measurements at various locations around your grow area and average them together. An ideal lighting environment would have equal PPFD all over your grow area, but this is simply unattainable and different lights have different light hotspots and distributions. In general, Cannabis can tolerate a very high level of PPFD. Certain studies have shown that Cannabis photosythesis rates can be maximized at PPFD levels of 1500-2000 PPFD [8, 9] at normal CO2 levels. However, many also seem to agree that indoors at temperatures around 25C, PPFD over 1000 provides a negligible yield increase unless CO2 is also supplemented. It is hard to know for sure without repeating previous experiments.

Based on common ‘rule of thumb’ knowledge:

• Most indoor growers should shoot for PPFD levels between 700-1000 across their grow space to maximize the photosynthetic rates of their flowering plants (though many have less).
  • 700 PPFD is generally sufficient for a grow area with a good spectrum, but many growers like to have PPFD levels of 800+ across their grow area
• Seedlings and clones grow well with PPFD levels of 100-200 PPFD
• Established plants in the vegetative stage do well with PPFD levels of 300-600. However, I often provide my vegetative plants with the same amount of light as my flowering plants (around 800 PPFD), though I would recommend around 600 PPFD for cost efficiency purposes. I think vegetative plants can utilize a good amount of light and it helps them acclimate to flowering intensities.

PPFD is a measurement of PAR

PAR (Photosynthetically Active Radiation)

This is the amount of light available for photosynthesis, which lies between 400nm and 700nm, as described by the McCree curve shown below. This is essentially a measurement of energy harnessed by the plant in relation to the wavelength of the photon hitting the plant.

However, the long-held beliefs about PAR are being challenged, particularly by Professor Bugbee at Utah State University, proposing that far-red (700-780nm) light is more photosynthetically active than previously thought, and can have an effect on plant morphology (stetching, elongation, cell enlargement), and may also have a role in helping flowers initiate more quickly.

For quite a while, most LED grow lights were comprised of an array of only two different colored lights, red and blue. The was because the lights were designed to target the absorption spectrum of chlorophyll, shown below:

However, as can be seen by the McCree curve, all visible wavelengths are also photosynthetically active. Some pigments called carotenoids have been characterized as photosynthetically active for wavelengths up to 550nm. While known photosynthetically active pigments may not absorb green light very well, it appears that chlorophyll a and b can utilize green light in small amounts.

Green light is particularly effective at penetrating leaf tissues. Up to 80% of green light passes through a chloroplast, and it is able to penetrate far into the mesophyll cell layers [1]. In whole, a leaf is able to absorb up to 85% of incident green light as the light is refracted within a large area of leaf tissue. Up to 10% of green light is able to penetrate through leaves to the lower canopy as well [2].

At very high PPFD levels of broad-spectrum white light, geen light can actually stimulate photosynthesis more efficiently than red light. This is because at high light levels (which Cannabis is normally grown under), chloroplasts in the upper mesophyll cells can become ‘saturated’. As red light continues to be absorbed, excess energy can actually be dissipated as heat. However, green light is absorbed less efficiently than red light by a given chloroplast, and so it penetrates further and is able to be absorbed by chloroplasts in lower mesophyll cells that are not saturated, [1].

It has also been noted that light wavelength has a direct effect on plant morphology. For instance, it is common for growers to use metal halide lights during vegetative growth because it has a more blue spectrum than high pressure sodium lamps, and blue light tends to promote shorter, more bushy growth patterns. HPS lamps are generally used in flower because the more red spectrum is more photosynthetic efficient and may help in promoting flowering.

HPS lights are more efficient to run than MH lights, and some hypothesize that red light may help stimulate flowering. While it has not been shown in Cannabis, red light has been known to accelerate flowering in some other species [6]. Green light may act as a shade signal when in a high enough ratio to other wavelengths, resulting in plants that may stretch or induce flowering early [7]. In short, all wavelengths seem to have some effect on morphology of the plant, and sometimes different wavelengths can act antagonistically or synergistically, meaning that it is not fully understood how different types of lights are affecting the phenotype of the plant both morphologically and metabolically. The spectrum of the sun is amazingly even, and so the different wavelength ratios of different grow lights likely have phenotypic effects that are not fully understood. A representation of the spectrum of sunlight is shown below:

PPE- Photosynthetic Photon Efficiency

This is a way to measure how efficient your grow light is at converting input energy to photosynthetically active radiation, measured in micromoles of PAR photons per Joule of energy. Some of the newest LEDs have an efficiency of over 2.0. A lot of the high quality LEDs from around 2014 were about 1.7 PPE, which at the time was comparable efficiency to double ended HPS lamps. A lot of LEDs of the time were actually less efficient than HPS lamps. I still have a couple blurple Mars 300 lights from around 2014 that are probably quite low on the efficiency scale.

How Do Different Grow Lights Work?

Fluorescent

Fluorescent lights are made of a glass tube under mild pressure. The tube is filled with an inert gas such as Argon and a small amount of liquid mercury. Electrodes at either end of the tube produce a voltage that sends electrons through the Argon gas from the cathode to the anode. This vaporizes the liquid mercury into the arc. Some of the electrons hit the mercury atoms, causing electrons in the mercury atoms to become excited to higher energy states. As the electrons drop back down to lower energy states, photons are emitted at UV wavelengths. Fluorescent tubes are coated in phosphor that shifts the wavelengths of the photons, resulting in a distribution of different wavelengths and a fairly broad light spectrum.

Fluorescent lights are the least useful lights on this list in my opinion. I would not use CFLs or T5s for anything other than rooting cuttings and/or small seedlings or maintenance of mother plants. They are not a bad choice for vegetative growth, but I believe metal halide and LED lights perform better and may be more efficient.

Most cheap LEDs that you would use to replace fluorescents for seedlings also tend to be composed of only red and blue diodes, which creates a very unnatural purple light. If you operate a dispensary keeping clones in a showroom for sale, the balanced spectrum from the fluorescents might be the better choice simply because it’s a bit more appealing to customers and the plants can be seen much better.

Because of the lower light output by fluorescents, they generally need to be kept pretty close to the top of your plants (around 6-10 inches).

LED

This paragraph will get a bit technical: LEDs (Light Emitting Diodes) are semiconductor light sources. This means that the conductivity of the material is in between that of a pure conductor and an insulator. Generally, there are three layers in an LED. The n-type semiconductor has an excess of electrons and the p-type semiconductor has an excess of ‘holes’, which are regions where electrons are absent in a crystalline lattice. These semiconductor types are made by adding different impurities to the materials. In between the two layers is an active layer that does not have an excess of electrons or holes. When a voltage is applied, excess electrons and holes from the n and p layers flow into the active layer, and when the electrons recombine with electron holes, they drop in energy states from the conduction band to the valence band. When this drop occurs, the difference in energy is released as a photon with a particular wavelength depending on the energy difference between the conduction and valence bands. The energy difference can be altered by changing the alloy composition. For instance, if the active layer is composed of indium gallium nitride, altering the ratio of indium nitride to gallium nitride can tune the bandgap of this material to different wavelengths of light. As you can likely guess, this means that each active layer can only emit a particular wavelength of light, which is why many grow lights will use a number of different colored diodes in order to provide light of different wavelengths to your plants. Broad spectrum LEDs can be made by coating blue LEDs with different colored phosphors, similar to fluorescent lights. This can shift the wavelength of blue photons, making emitted light come out in a broad spectrum of wavelengths. The typical phorphor coated LED spectrum looks something like this:

Two major LED types are used in LED grow lights: SMD and COB.

SMD- Surface mounted diodes

large 5x5mm SMD LEDs are capable of housing up to 3 LEDs per module. These are diodes that do not require wires. Instead, they are soldered directly to a circuit board. Sizes vary greatly, from under 1mm squared to over 25mm squared. Most SMD circuit boards are 3W or 5W and are composed of 0.5W modules, although 10W SMD circuit boards do exist. Each diode in an SMD module has its own anode and cathode. Due to these properties, a variety of spectrums can be produced by SMD panels that can span the PAR spectrum.

COB- Circuit on Board

COB LEDs allow for multiple diodes (at least 9, but can be much higher) to be attached to a single substrate with a single circuit (one cathode and one anode). Diodes can be placed quite close to one another and produce a high light intensity for a small space. Most COB grow lights will have a mixture of warm white and cool white (3000K-4000K) diodes, resulting in a wide and benefiial spectrum for plants. Both COB and SMD LEDs are quite efficient, though a panel of 3W SMD LEDs is generally slightly more efficient than COB LEDs. COBs tend to have a much more focused light than SMDs, meaning that for the light spread to be useful over a large area, a lens usually has to be placed to increase the area that the light can reach.

HID Lights

HID stands for high intensity discharge. Like fluorescent lights, HID lights are arc lights. They work by forming an electric arc between two tungsten electrodes that are housed in an arc tube. The arc tube contains a noble gas that ignites when the circuit is active. The gas begins to heat up and eventually vaporizes metals within the arc tube. Light is produced when excited electrons drop in energy levels, releasing photons. The spectrum of the HID light is largely determined by the metal and impurities that are being vaporized in the arc tube. There are three main types of HID lights used in plant cultivation: HPS (High Pressure Sodium), MH (Metal Halide), and CMH (Ceramic Metal Halide).

HPS

As you can probably guess, HPS lamps contain sodium as the primary metal in the arc tube. Other impurities such as mercury are added to alter the spectrum of the lamp to have more blue wavelengths. HPS lights are dominated by orange, yellow, and red wavelengths, while still containing small amounts of blue and green wavelengths. The composition of the materials in HPS bulbs for horticulture has changed over time to try to make a more balanced spectrum. An HPS spectrum looks something like this (although each lamp is slightly different):

Double ended HPS bulbs began to be popular for horticulture around 2014. Double ended HPS bulbs are slightly different because they don’t screw into a socket, they have electrodes at either end of the bulb similar to fluorescent tubes. DE HPS lamps, as mentioned previously, are more efficient than their single ended counterparts. For SE HPS bulbs, there are 4 common wattages, 250W, 400W, 600W, and 1000W. 600W bubs are the most efficient, while 1000W bulbs are second most efficient. 400W is less efficient still and 250 W bubls are the least efficient. As many long-time indoor growers know, HPS bulbs used to be the only way to grow indoors with high intensity light and they have been the gold standard of horticultural lighting for decades. Even today, HPS bulbs are some of the best flowering lights you can buy, though modern LEDs are quite competetive and arguably better. Because of the high amount of red light, HPS bulbs are generally only used for flowering. The red light of the HPS affects the structure of cannabis (during vegetative growth) causing it to become more lanky and weak in stem and branch growth.

MH (Metal Halide) MH bulbs also contain mercury, though in a higher concentration than HPS bulbs. However, MH bulbs have metal halides instead of elemental sodium as the light source. Metal halides are metals that are compounded with bromine or iodine. A MH spectrum might look something like this:

As you can see, there is very little red and far-red light in this spectrum as compared to the HPS bulb. This not only reduces the photosynthetic efficiency, it has an effect on plant structure, causing the plant to be more compact than it would under a red-light heavy spectrum. There is also very little far-red light.

CMH- Ceramic Metal Halide

CMH lights are the newest HID lights used in horticulture. They are more efficient than MH bulbs and also have more red in the spectrum. In fact, the CMH spectrum is fairly well balanced and could be compared to a mixture of MH and HPS bulbs. For this reason, they can be quite effective at both vegetative growth and flowering, growers don’t need to worry about switching out bulbs or even replacing bulbs as frequently. An example spectrum from a 3100K CMH bulb is shown below:

CMH bulbs also have a much longer lifespan than MH and HPS bulbs, and they produce far less heat than HPS bulbs. However, they are significantly more expensive than other HID options. Also, there is no 1000W CMH option; most of the CMH bulbs on the market are 315W or 630W double ended bulbs. Many believe that CMH bulbs may not be as effective as DE HPS bulbs in flower, but this is hotly debated. It is also important to look at the quality of the flower produced. CMH bulbs produce relatively more UV radiation and blue light, for example, which has been shown to be effective at stimulating the production of secondary metabolites such as cannabinoids and terpenes [10]. In the end, MH/HPS bulbs and CMH bulbs are both great choices, and each option has its positives and negatives. In fact, some growers will use both strategies in their grow area in order to take advantage of the broad spectrum of the CMH while also adding in supplemental blue-dominant and red-dominant light at different growing stages. One could also consider using HPS bulbs and supplementing with LED lights with a cooler color temperature to try to get a broader spectrum than HPS could provide by itself.

So, now that we have covered the basics of the different types of horticultural lights, how do you determine what to buy? Well, there is no right answer. If you spend any time on internet forums, you will soon realize that many different people feel quite strongly about their choice in light. My personal opinion is that it is best to mix different types of lights in order to reach the PPFD levels that you need at a spectrum that is acceptable. If you have a light or light mixture with a truly broad spectrum like a CMH bulb or broad spectrum LED, you don’t have to switch out lights for flowering. If you do not, it is best to stick to blue-heavy spectra for vegetating and red-heavy spectra for flowering.

How Expensive are Different Lights?

Cheapest (<$200/ sq. ft.)

If you are on a tight budget, I recommend a single-ended HID setup using a digital ballast that can support both MH and HPS bulbs (MH for veg, HPS for flower). This can cost less than $200 to light up 3 ft x 3 ft grow area, and you can even light up a 4 ft x 4 ft area using HID for less than $200, though I would not be comfortable doing the same with LEDs at this price point.

Mid-Range ($200-$400)

If you have a moderate budget ($200-$400)/ 10 square feet, and don’t want to have to worry about switching bulbs out, CMH lights are a very good option. DE HPS is a very good choice for flowering as well, and there are fixtures that can also run DE MH bulbs for veg. While CMH lamps can be quite good, some argue that they need some supplemental red light to help get the most out of flowering. CMH lights may be more efficient overall than a MH/HPS system since MH lamps tend to be less efficient than CMH and HPS lamps.

This price range also allows for some nice LED choices. For instance, the Mars Hydro TS3000W light is a good price for the light, but it uses some cheaper materials and designs for the fixture. It covers a 4 ft x 4 ft quite well and has a broad spectrum with some supplemental red light (SMD chips) and can be purchased for $480.000, giving an average cost of $27.5/1 square foot. There are also some cheaper COB and COB/SMD combo grow lights that can be purchased within this price range, though the COB LEDs used in these cheaper lights are usually either not a high quality full spectrum COB or they are not as powerful over a large area compared to SMD panels. A good 100W COB LED can be purchased at this price point, but it will not light up a 10 sq. food area well at all.

Big Budget ($400+ for 10 sq. feet)

This is the price range in which LED really shines. As I said, there are a lot of opinions out there about if LEDs really can outperform HID lights. However, a lot of people simply don’t have physics on their side. There is no magic to HID lights that make them the be-all-end-all of grow lights. If you have LED lighting with the exact same PPFD, the exact same distribution, and the exact same spectrum as an HID light, they will perform exactly the same. HID lights do not ‘penetrate’ better. Photons travel at the same speed regardless of what their source is, and in fact, the most important wavelengths for canopy penetration are green wavelengths. A well-designed full spectrum LED light will have more green light than HPS lights. Therefore, it is actually modern LEDs that have better penetration than HPS lights (although CMH lights tend to have good amounts of green light as well). Furthermore, no grow lights on the market can match the efficiency in PAR/Joule of the top tier LED grow lights.

For instance, for $1,000.00, you could purchase a Chilled logic 660 LED grow light, With an efficiency of around 2.4 PAR/Joule, an even distribution, and a broad spectrum with supplemental red wavelengths that looks like this:

it is hard for HID lights to compete. The measured PPFD is similar to HID lights as well. The PPFD measurements over a 4×4 area are shown below:

I have not used this light, it is just one that I saw mentioned recently that I am using as an example of the kind of performance one can achieve with modern LED lights.

UV Lighting

Finally, it is important to consider whether or not you want to include UV light in your grow. For instance, UVB has been shown to increase cannabinoid levels in Cannabis [3], and many CBD and marijuana commercial growers are beginning to add UV light to their grows (specifically UV-B and UV-A).

UV-B (290nm-320nm) light appears to be sensed by the UVR8 photoreceptor and plays a stimulating role in producing secondary metabolites such as cannabinoids and terpenoids [3,4,5]. Essentially, UVR8 induces upregulation of genes involved in producing metabolites involved in defense against various environmental and biotic factors including sunlight. It is difficult to achieve the extremely high 30% THC levels found in some buds today without supplemental UV lighting. UV-A light has also been shown to increase cannabinoid levels [10], and does not cause DNA damage to cells. In fact, UV-A and far-blue light can activate DNA repair mechanisms through a process known as photreactivation that can mitigate some damage from UV-B [11]. UV-A is sensed through different proteins than UV-B including cryptochromes and phototropins and acts to repair DNA damage by upregulating the expression of photolyase proteins [12]. In the proper ratios, UV-A exposure can enhance plant response to UV-B light. Though UV-A exposure may help with secondary metabolite production, stimulation of the UVR8 pathway is very important to maximize the effects of UV light. In actual sunlight, UV-A is present in greater amounts than UV-B, and obtaining the proper ratio will be important for optimizing plant UV responses for the purpose of cannabinoid production. Most UV lights on the market do not have the same ratio as sunlight, but I believe it is important to at least make sure that the lights chosen have both UV-B and UV-A represented in the spectrum.

UV-B light is damaging to cells, and cannabinoids act as a suncreen, absorbing UV light in trichomes before the light reaches plant cells. This explains why UVR8 may participate in upregulating cannabinoid production. The best way to currently supplement plants with UV light economically is with UV T5 fluorescent bulbs, which can be purchased quite cheaply, and will provide both UV-A and UV-B light. Reptile UV lights and similar short-tube designs are not strong enough, and UV LED lights are too expensive simply for a supplemental light.

It is difficult to suggest how long to leave UV lamps on or when to start supplementing, as many people suggest pulsing the UV light for short periods of time, using them every other day, or only using during the last couple weeks of flower. However, many growers also advocate using them daily for multiple hours a day, even during vegetative growth (sometimes up to 12 hours per day). For instance, Lydon et al. 1987 found increased cannabinoid production after 40 days of exposure, and cannabinoid concentrations increased with dosage. In nature, UV radiation is present whenever visible radiation is, so it makes sense that the UV-B and UV-A radiation from a T5 won’t be too much for the plant to handle. I believe more optimization experiments are needed here, but I would say that my current recommendation would be to use daily, at least during the flowering cycle, for at least 4 hours per day, and up to 12 hours per day should be okay but the returns from longer time periods are likely diminishing. The one time I might cut back on UV exposure is during the last couple weeks of flower, after which cannabinoid production won’t increase by a large amount, but UV radiation might contribute to cannabinoid degradation. I would probably limit the expose to about 2 hours per day during the last couple weeks of flowering.

Product Recommendations (In no particular order): I will provide recommendations based on reviews and tests I have watched and read online, not based on testing all of these products personally. Recommendations are not ranked.

Fluorescent:

T5- With T5s, one can get a variety of fixtures and bulbs of different lengths, color temperatures, and number of bulbs.

• Complete Kit (value): DuroLux T5 Grow Light (4ft 4lamps) DL844s Ho Fluorescent Bulbs- ($92 on Amazon)
• Improved Spectrum LED T5s full kit: Active Grow with 8 x 24W T5 HO 4FT LED Tubes ($385)
  • While this is better than standard T5s, it is overpriced compared to SMD or COB LEDs at similar watts in my opinion
• Recommended fluorescent bulb for highest performance: Hortilux PowerVEG Full Spectrum with UV 54w – 4ft T5 HO Bulb ($28)

UV

For UV supplementation to any grow light system, I recommend using AgroMax Pure UV T5 bulbs.

• 2 ft.- $22
• 4 ft.- $28

Alternatively, the Hortilux PowerVEG proveds plenty of cool-white PAR as well as UV that would be a good supplement to HPS bulbs. ($28/bulb)

HID

In regards to HID fixtures, I think Vivosun and iPower are both good products that are a good value. I have had the iPower 600W ballast for around 6 years, and it is still running well.

HPS/MH– Keep in mind that different bulbs may have different color temperatures. For CMH, 3100K is a good color temperature in my opinion.

DE HPS/MH Fixtures (good values, good performance)

• iPower ($259)
• Vivosun ($226)

DE HPS Bulbs:

• Best Value- Ushio brand ($74)
• High Performance- Eye Hortilux ($94)
• I would recommend the same brands for MH bulbs

Single-ended HPS/MH

Fixture

• iPower 600W setup with wing reflector fixture ($120)- This is not air cooled, meaning that it will significantly heat your growing space, but will have greater efficiency since light does not pass through a layer of glass.
• iPower 600W setup with air-cooled hood fixture ($150)

Bulbs

• Performance: Eye Hortilux 600W HPS bulb- ($78)
• Best Value: Ushio 600W HPS bulb Optired- ($55)This is the bulb I have. I like it quite a bit and lasts quite well

1000W HID Fixtures and Bulbs:

600W Fixtures and Bulbs:

CMH

• HydroCrunch 630W DE CMH setup ($355)
• Eye Hortilux 315W CMH setup ($463)- high quality and has a higher UV ratio in the spectrum than many CMH bulbs.
• Vivosun 630W CMH fixture($230)- fits 2 315W bulbs
• Eye Hortilux 315W CMH bulb ($92)
• Ushio HiLux Grow 315W bulb ($82)

LED

High Performance

• I mentioned the Chilled Logic 660 earlier, and I would recommend this light for a 4×4 space, but it will work in a 5×5 space, especially for a home grower. It uses SMD LEDs.
• The Lumatek Zeus 600W is a fantastic SMD-based light with an extraordinarilly high efficiency of 2.3 umol/J. Like the Chilled logic, I recommend it for a 4×4 area with CO2 supplementation, but for a home grower it will perform great in a 5×5 area.
• The California Lightworks 500W SolarXtreme is a fantastic broad spectrum COB-based fixture with a high-quality fixture. I recommend it over their red/blue SMD light, the SolarSystem 550
• The Mammoth Lighting SMD 800W strip-lighting fixture can easily flower a 5×5 area with an efficiency of around 2.12 umol/J and costs around $1,000.
• The HLG 550 V2 R-Spec quantum board can easily flower a 4×4 area and has a claimed efficacy of 2.6 umol PAR/J, though I am sure the actual PPFD/J is lower than the PAR/J figure. It will cost around $850
• The Fluence SPYDR 2x and 2i lights are well known among commercial cultivators as being high quality and well designed for rack mounting. They have a claimed PAR efficiency of 2.7 umol/J, which again, likely doesn’t count the actual PAR landing on the plants.

Cheap but good

• The Mars TS3000 is a great deal for home growers looking to light up a 4×4 area well and a 5×5 area decetly. It will run you about $440. The Efficiency is claimed to be around 2.2 umol/J, though the PPFD/J is likely lower than this.
• The Spider Farmer SF 4000 will supposedly flower a 5×5 area quite well, has a claimed efficiency of 2.7 umol/J, and will run you about $570.
• For a 2×2 area or supplemental lighting for HID lighting, a 100W CANAGROW CREE CXB3590 is a good choice and is lensed to give a wider range. It will run you around $150.

Of course, there are many other products that will work well. These are just products I have seen reviews on that are well-received.

Combinations

It is never a bad idea to mix and match lights in order to improve the light spectrum your plants are exposed to. Some combinations might look like this:

• HPS bulbs for flower, supplemental cool temperature LED lights to increase blue and green light, and UV T5 bulbs
• CMH bulbs for flower, supplemental red warm temperature LEDs to help with flowering, and UV T5 bulbs
• Broad Spectrum LED lights with UV T5 bulbs

If you are doing combinations, you will want to slightly increase the spacing of your main light fixtures and to install supplemental lights in between your main fixtures. Make sure to space your lights so that your PPFD in any given area is not too high (wasting energy). Do not change spacing if only adding UV bulbs as these do not contribute to PAR.

Hopefully this has answered some questions!

1. Terashima, I., Fujita, T., Inoue, T., Chow, W. S., & Oguchi, R. (2009). Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green. Plant and Cell Physiology, 50(4), 684–697. https://doi.org/10.1093/pcp/pcp034
2. Growing Plants with Green Light – Greenhouse Product News. (n.d.). Retrieved February 22, 2020, from https://gpnmag.com/article/growing-plants-with-green-light/
3. Zavala, J., & Ravetta, D. (2002). The effect of solar UV-B radiation on terpenes and biomass production in Grindelia chiloensis (Asteraceae), a woody perennial of Patagonia, Argentina. Plant Ecology, 161, 185–191. https://doi.org/10.1023/A:1020314706567
4. Lydon, J., Teramura, A. H., & Coffman, C. B. (1987). UV-B RADIATION EFFECTS ON PHOTOSYNTHESIS, GROWTH and CANNABINOID PRODUCTION OF TWO Cannabis sativa CHEMOTYPES. Photochemistry and Photobiology, 46(2), 201–206. https://doi.org/10.1111/j.1751-1097.1987.tb04757.x
5. Liu, H., Cao, X., Liu, X., Xin, R., Wang, J., Gao, J., Wu, B., Gao, L., Xu, C., Zhang, B., Grierson, D., & Chen, K. (2017). UV-B irradiation differentially regulates terpene synthases and terpene content of peach. Plant, Cell & Environment, 40(10), 2261–2275. https://doi.org/10.1111/pce.13029
6. What Effect Does Red Light have on Plants? | Ursa Lighting. (n.d.). Retrieved February 23, 2020, from http://ursalighting.com/effect-red-light-plants/
7. Zhang, T., Maruhnich, S. A., & Folta, K. M. (2011). Green Light Induces Shade Avoidance Symptoms. Plant Physiology, 157(3), 1528–1536. https://doi.org/10.1104/pp.111.180661
8. Chandra, S., Lata, H., Khan, I. A., & Elsohly, M. A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO. Physiol. Mol. Biol. Plants, 14, 4.
9. Chandra, S., Lata, H., Mehmedic, Z., Khan, I. A., & ElSohly, M. A. (2015). Light dependence of photosynthesis and water vapor exchange characteristics in different high Δ9-THC yielding varieties of Cannabis sativa L. Journal of Applied Research on Medicinal and Aromatic Plants, 2(2), 39–47. https://doi.org/https://doi.org/10.1016/j.jarmap.2015.03.002
10. Magagnini, G., Grassi, G., & Kotiranta, S. (2018). The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L. Medical Cannabis and Cannabinoids, 1(1), 19–27. https://doi.org/10.1159/000489030
11. Britt, A. B. (1995). Repair of D N A Damage lnduced by Ultraviolet Radiation. In Plant Physiol (Vol. 108). http://www.plantphysiol.org
12. Aphalo, P., Albert, A., Björn, L., Mcleod, A., Robson, T., & Rosenqvist, E. (2012). Beyond the Visible, A Handbook of Best Practice in Plant UV Photobiology.