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Lighting the Way Through Cultivation: A Q&A With David Hawley

By Leo Bear-McGuinness

Published: Mar 17, 2020   
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Coming out of the dark days of prohibition, cannabis cultivation is in need of some enlightenment. How much light should be provided to the plants? Where should it be directed? What bulbs should the industry use? Luckily, scientists like David Hawley are here to shed some light into this hazy field. As the head of research and development at the horticultural lighting company Fluence, Hawley has conducted several studies into cannabis lighting – the results of which have filtered down into the company’s products and practices.

Leo Bear-McGuinness (LBM): What kind of light is best for cannabis plants?

David Hawley (DH): It really seems like you need all parts of the spectrum. There are different colors that'll drive different things in plants, and whether that's just driving photosynthesis to get a lot of yield and big plants or whether you're trying to drive a certain flavor or aroma, you would use different parts of a spectrum. But, ultimately, the best overall spectrum tended to be something based more or less on broad spectrum light, like white light.

LBM: Are the LEDs always focused towards the top of the of the plant, and the buds specifically?

DH: That's one that we're trying to figure out. So conventionally, yes, the objective is to have fairly uniform light reaching the top of the canopy. So when we sell a light to a customer we’ll say, ‘OK, you want your lights mounted this distance away, and you want a light every meter or every 10 meters or whatever it is.’ However, that isn't necessarily the way of the future.

Some companies, us included, are exploring options of lighting the top and from the bottom of the canopy and from the side, because if you put all your light at the top and you have a very dense canopy, the quality of light will be very different at the top than it is at the bottom. In general terms, red and blue light [are] much more readily absorbed by plants than green light. And so you end up having a higher fraction of green light lower in the canopy. And that isn't necessarily a bad thing. It just means that your chemical profiles of the top of the canopy and the bottom will be different.

This is something that I did during my PhD. We explored supplemental sub-canopy lighting and we found that we did get better bud uniformity if we had lighting at the top and the bottom of the canopy. What's more is that lighting from the bottom of the canopy significantly increased the yield. In this case, we had added about 15 percent more light and we got something like 19 percent more yield back, which is really interesting. It just means that the plant is really utilizing that energy much more efficiently than it was would.

LBM: Do different cultivation settings – whether they be indoor setups, vertical growing areas, or greenhouses – necessitate different lighting set-ups?

DH: Yes. So, particularly in North America, the emphasis so far has been on indoor production. And the strategy for indoor seems to be split between doing a broad bed of plants in a single tier or vertical systems, [which are similar to] Japanese plant factories, which have racks and racks of lettuce plants. Vertical systems are like a scaled down version of that; you might have two or three tiers of cannabis plants. And with that, certainly the lighting strategy is very different, because you can't just have one light at the top of your room; the upper cells will be blocking the lower shells from receiving any light. And so that's where LED technology really shines, if you can forgive the pun.

The fact is that LEDs have really enabled this vertical [growing] that you just couldn't have done before with any of the older technologies, for instance, high pressure sodium (HPS) lamps. They're these big balls and they throw off a lot of heat. And if you were to try and get a bunch of those into a vertical system, it would just get way too hot, and it would be really difficult to manage. Whereas with LEDs, you have these fixtures that have a really slim profile. And so you can integrate them really easily into racking system and they don't really take up much space and they don't really produce that much heat. So if you have the space for it, and you have sort of the technical know-how, this is generally a better way to go because if you can tackle more plants with a smaller footprint.

LBM: So LED bulbs could help the industry's carbon footprint?

DH: Oh, absolutely. I mean the gap in efficacy between LED and older technologies like HPS continues to grow. Certainly depending on the spectrum that you use with LED, your efficacy can range from the two micro moles per joule, which is on the low end, all the way to 3.5, which is kind of the upper limit of what's in the space right now. Regardless of what spectrum you use for LED, it's still going to be much more efficacious than what you would see with HBS.

LBM: But would certain states, such as California, benefit more from outdoor growing? And should other states with less light throughout the year, such as Massachusetts, focus on these LED systems?

DH: That’s a question we spend a lot of time thinking about. So there's a rule of thumb where if you give plants 10 percent more light, you will get 10 percent more yield back. And if you give them 20 percent more light, you get 20 percent more yield, and so on. Until a point it's directly proportional. And for something like lettuce, that doesn't remain true for very long. Whereas with cannabis, we haven't seemed to have found the ceiling yet for too much light. That does vary a little bit with cultivar and with the genetic line of cannabis. But it seems that if you have a really high-light environment, that's good. And so what that means is that if you're at a more southern latitude, and you have the option of growing in a field with a lot of sunlight, that's going to be a good option.

[But] if you're a little bit more northern, up into Canada or in the Netherlands where it's dark, you're going to be supplementing quite a lot of light. And if you move to a system like that, that's less controlled, that's not necessarily ideal for the cannabis space right now. The best possible thing to do is just try and produce very consistent plants, because at least if you have consistent plants clinical research can happen, where we'll begin to understand how these chemical profiles actually do affect people. And if you move to field agriculture it's very hard to get consistent plants. Right now anyway, the market really would prefer to have much better control over those profiles.

This article originally appeared in Analytical Cannabis' Advances in Cannabis Cultivation Science ebook in December 2019. 

Leo Bear-McGuinness

Science Writer & Editor

Leo joined Analytical Cannabis in 2019. From research to regulations and analysis to agriculture, his writing covers all the need-to-know news for the cannabis industry. He holds a Bachelor's in Biology from Newcastle University and a Master's in Science Communication from the University of Edinburgh.


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