2019 review of research on indoor cannabis growing

Indoor Growing
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This paper is meant to report much of what had been established experimentally about growing pot indoors, as of 2019. I’ve summarized the findings that are relevant to non-commercial grows.

Jin, D., Jin, S.X., & Chen, J. (2019). Cannabis indoor growing conditions, management practices, and post-harvest treatment: A review. American Journal of Plant Science, 10, 925-946. Cannabis Indoor Growing Conditions, Management Practices, and Post-Harvest Treatment: A Review

Light spectrum

Research on spectral effects has shown that LED lighting produces smaller, more compact plants than HPS, reducing yield by 12%. This reduction has been attributed to the increased red and blue energy LED lights produce, but may be due instead to a cooler growing temperature because LEDs produce less heat.

THC yield can be increased by including UVA. In one study, an LED w/UVA produced 15.4% THC, vs. 9.5% for HPS (a 62% increase).

PPFD, temperature, and CO₂

The authors discuss the Chandra et al. papers on this topic that I’ve summarized at Temperature, PPFD, & CO2: Some research findings .

They also summarize two studies that show yield is unaffected by the number of plants in a grow area. Once the area is filled by the canopy, the yield per plant decreases proportionally as plants are added. These findings support using watts per ft² rather than watts per plant to estimate lighting needs.

Light cycle during flowering

The standard 12/12 schedule for flowering photoperiod plants is best. A study showed that plant and cannabinoid yield (measured in grams/m²) were higher under a 12-hour-on cycle versus 11; increasing to 13 produced no benefit.

Humidity

I was surprised that only two studies are mentioned. The more useful one reports that 75% RH is best for seedlings and 55 - 60% is best for the subsequent stages.

Harvest Timing

Four criteria are suggested for deciding when to harvest: (1) The percentage of brown stigmas reaches 75%; (2) The buds are firm when squeezed; (3) The color of the trichomes matches the grower’s intent; and (4) The aroma reaches its peak.

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Thanks for summarizing all of this. I will dive into the links.

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@merlin44 Please have a look at Nutrient feed rate calculator, while you’re at it.

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Thank you, will do…

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5 years old data may as well be Egyptian hieroglyphics. LED’s blow everything else away.

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We have a better understanding now of how and why LEDs perform as they do. That advance doesn’t invalidate the entire review, nor does the age of the data. What’s changed is we have more data on LEDs now and understand it better.

It’s also important to bear in mind that the review is restricted to findings that had been published in peer-reviewed scientific journals. “Common knowledge” tends to run ahead of science and is sometimes the motive for formal experiments. Sometimes, the common knowledge turns out to be correct, sometimes it’s partly correct, and sometimes it’s wrong.

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My only thought is a combination of HPS and LED is where its at. Especially in Winter.

I say that from experience not a book. The ones next to the LED get sho nuff frosty but lack the weight, introducing MH and HPS in the middle solves the issue.

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The issue stated is light spectrum and yet somehow became synonymous with leds.

Myth busted as not repeatable.

I like studies, they just need to be good studies if they’re going to draw these kind of conclusions.

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That was my doing.

For UVA? Perhaps you’re thinking of a recent discussion about UVB here: Cultivating Chaos with Madam Calamity & Low - #1055 by Low

Low produced a 2024 Master’s thesis by Matthew Talin that confirmed prior findings that UVB has no benefits for cannabis.

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There are contradictory studies for both uva and uvb. Part of science is being repeatable and 62% increases are not. Someone is probably going to get where this probably has something to do with genetics. Maybe they have already. But i believe increases have been shown with violet and royal blues which would be up to about 460nm. Until these are sorted by cultivars and individual wavelengths it’s really hard to write these down as fact.

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It’s important to distinguish between findings and conclusions. The UVA study that was cited got a 62% increase. That’s a finding, i.e., fact.

The conclusion the authors reached was that UVA can increase THC yield. That’s not a myth that’s been debunked.

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Agreed. But saying like that completely devalues the finding. Uva can increase thc yield. Violet can increase thc yield. Blue can increase thc yield. Increasing photon count of the same light spectrum can increase thc yield. It’s not really relevant information at that point.

In order for something to be a scientific fact it needs to be repeatedly comfirmed. So every study introducing uva should provide similar results. Since neither their findings or conclusions are repeatable the entire study is relatively useless.

Despite a lack of contemporary published scientific studies on the effects of UV exposure on cannabinoid content (Magagnini et al., 2018), there is a popular belief that UV exposure can substantially enhance cannabinoid content – particularly THC – in inflorescence tissues in modern cannabis genotypes. Genotypic predisposition to producing THC is also an important consideration since inflorescence THC content may be many times higher in modern vs. older cannabis genotypes (Dujourdy and Besacier, 2017). Therefore, genetic factors may play a more significant role in altering inflorescence THC content than environmental stresses such as UV exposure.

The only safe position on this is that more data is needed.

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Saying that UVA can increase THC yield is the only defensible conclusion re. UVA from that study. That conclusion merits additional experiments to learn what variables determine whether UVA produces an increase and how big the increase is.

No. The 62% increase that study obtained is a scientific fact, i.e., a finding from a scientific experiment. If multiple studies obtain similar increases, we can conclude it’s likely (not proven) that UVA used in the same way will produce roughly a 60% increase for others, too.

If a new study then gets no increase and there’s no apparent flaw in the experimental procedure, we know the original conclusion is too broad. That calls for further experimentation to determine why no increase occurred, so we can refine the conclusion.

The 2022 paper you linked got a “no increase” outcome. That doesn’t debunk the benefits of UVA. It leaves scientists with the task of determining the circumstances under which UVA is and isn’t beneficial.

On another topic: I’ve been searching out scientific papers dealing with growing cannabis. When I find ones I think will be of interest to our community, I post summaries like this one. I include a link so anyone can read the paper themselves and post remarks.

I’m collecting articles dealing with the effects of UVA. I have the two referenced in this thread – pointers to more are welcome.

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Please do post as you see anything. Articles are all over the place right now with red/infrared, uva/uvb from what ive read in regards to growth/thc. I dont want to invest in any of it until there is a bit more data on it. Vendors make it all out like they are the silver bullet you need for your grow. Just goes on and on, like migro claiming uva does nothing but their uvb bulb can increase terpines by 10% or so, with many vendors claiming their supplimental uva lights increase terpines. I just dont want to be the money wasting guinea pig. Cheers.

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I disagree, otherwise it would be repeatable. It should be this way, but they clearly didn’t do enough to isolate variables. Which is why i will continue that this information is completely useless. You can take it for whatever you want, but I put no value into it.

To me, this is like saying using superthrive will grow you 10 foot plants. Then showing a picture of a plant that has extremely long internodal spacing and is 10 feet tall. Someome who is familiar with how this happens will know it had nothing to do with superthrive nor is the plant in question desirable. But it could be proven that the plant is 10 feet tall and superthrive was used.

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We’re having a terminology problem.

By definition, a scientific fact (as opposed to other types of facts) is an outcome from a scientific experiment. The 62% increase satisfies that definition.

I described previously how, if further experiments produce similar results, we gain confidence that they’re generalizable. That’s where repeatability becomes relevant.

What variables did they neglect?

Yes, but that’s not an outcome of a scientific experiment. To meet that criterion, there would have to be a control condition, i.e., plants that didn’t receive the treatment but were otherwise grown under identical conditions. I say “plants” because an experiment that compared only two wouldn’t be convincing, for obvious reasons.

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Exactly, but the repeatability is an equally important part of scientific method. An individual shouldn’t get to decide which of these is important and which aren’t.

I don’t care enough about uva or thc content to continue posting about it. I just hope that unsuspecting readers don’t scroll by thinking they’ll see a 62% increase in thc content because they drop some cash on uva equipment.

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Thanks for the post @Venturi and the links. Thanks @dbrn32 for the back and forth dialog. I dont know enough to have an opinion either way but I love reading stuff like this.

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Interesting debate. Nicely done and adult. So refreshing. :grin: I have a strong opinion myself on uv stressing. 1 thing i DO believe is a scientific fact is not a fact till many many repeats are done with an average result of the same thing allowing maybe 5% for uncontrolled or unknown variables. We are finding the mathematical facts we thought true are being updated with new information available same as most gravitational and quantum theories we thought we had down solid. Since light is only the energy provider I think closer look into actual plant changes vs light changes will give a better idea of how to get higher compound concentrations. We have crossbred so many different strains from so many different climates it is a turkey shoot on what happens. All chasing thc production.
Tropical cannabis in a sea level environment many under canopy, vs a mountain strain with thinner air/more uv damage resistance. Where is the median for what any strain today can take? I feel if I hit genetic that just can’t take uv from evolutionary defenses not in the genetic do i really help my grow and result or hurt it? We bank on the fact that stressors increase thc production as a defense but is uv really the 1 we want? I am an on the fence till it is proven but not a hater. I am not a big studier of the science side, lazy cave man style, but since we breed for high trichome production and the thc is a process of the trichomes lifetime, does the stressor give us an ROI that is worth the damage we can do? 62% increase is a big number. Not hating or being a smarty pants but where do we draw the line on the chase? Are we considering the terpenes which can’t take high temps and the effect on that very important medical part of this God given sedative? Just pointing out that thc is not all this plant is about anymore, and I would LOVE to see a study adding in loss or increase of other chemical compounds in the med when using a uv stressor that actually kills everything on this planet. Sorry for butting in but I am the less is more guy. Yes that guy. I personally truly think most of our gear suggested and brainwashed in is just marketing. 40 years retail mostly management showed me a lot of make money products not needed in the least. Just the minimalist view. Looking forward to you guys getting some good answers with all the conflicting info. The truth is there… :grin:

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You’re not alone. Who is asking what the practical difference between weed testing at 9.5% and 15.4%? If 3 pulls of the latter gets you a nice buzz, does it require 4 pulls of the 9.5% sample? Maybe 5? This is not even considering other cannabinoids and terpene profiles that have been trampled in breeding efforts seeking higher thc content.

Personally, I’ve never been able to distinguish the thc content of anything I’ve smoked. And with labels provided from dispensaries I can say that I’ve gotten more high on weed testing at 18% thc than weed testing at 30% thc. But we already know that other cannabinoids and terpenes play a major role on individual experiences.

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