All the pretty lights in cannabis cultivation

Due to the desired growth habits of Cannabis sativa, lighting may be more critical for this crop than many others. Even before it became legal again, growers were working on fine-tuning their lighting set-ups to achieve the perfect plant-to-flower ratio.

Both public universities and private researchers have been looking into this balance, and both sides presented some of their information at the recent American Society for Horticultural Science conference. Fluence, an Austin, Texas-based business with advanced LED lighting solutions for controlled environment cannabis cultivation, led two sessions on the topic at the conference.

Looking at “the promises and pitfalls of dynamic lighting for cannabis cultivation” was Fluence’s David Hawley, Ph.D.

Dynamic lighting (adjusting the light spectrum and intensity throughout the plants’ lifecycles) has evolved from theoretical research to commercial viability, according to Hawley. This style of lighting can help produce quality crops while maintaining cost efficiency, if done correctly.

“Technology has gotten to the point where automation can really improve quality and yield – but cannabis has a long way to go,” Hawley said, mostly due to a lack of research and the fact that plant genetics and cultivation practices are “all over the place.”

So Fluence is studying ultraviolet (UV), blue, red and far-red light and how they influence cannabis inflorescence quality and yield.

Lighting questions that growers brought to the business included “Can UV-B enhance potency?” “What about blue?” “What about ramping PPFD to emulate sunrise and sunset?” “Do I need far-red?” “What about the Emerson enhancement effect?” “PAR or ePAR?”

(For reference, PPFD is photosynthetic photon flux density – how many essential photosynthetic photons are actually impacting the grow area and how well lights are working when it comes to their output. The Emerson effect is the increase in the rate of photosynthesis after chloroplasts are exposed to light in the deep red spectrum. PAR is photosynthetic active radiation, the wavelengths of light within the visible range and which are necessary for photosynthesis. ePAR includes the non-visible wavelengths, which have lower effectiveness. UV-B lights emit a spectrum of ultraviolet light with wavelengths even lower than ePAR.)

According to P.L. Light Systems, “while red is most efficient for photosynthesis, having only red light would result in poor growth like very elongated stems. Blue is added to keep plants compact and a more typical shape. On the other hand, you can’t grow plants under only blue light, as their growth and development will be adversely affected.”

So, there’s a lot involved in cannabis lighting. What did Hawley and his team find out?

“The more UV-B used during flower, the worse things grow,” he announced. “But I do think there’s opportunity for it” – specifically, hardening off during the plants’ vegetative stages.

Blue light was found to be good for anthocyanins (both for pleasing plant color and antioxidant values), but not for much else.

For those intrigued by far/deep red lighting, Hawley said the Emerson effect isn’t currently relevant in commercial cannabis production.

“Red represents the biggest cost-saving opportunity with lighting,” he concluded. Even better for growers, red lights tend to be cheap to build, buy and use.

High & Low Red Content Lighting

Hawley was joined by Fluence Research Project Manager Brian Poel for an evaluation of high and low red-content lighting during the vegetative phase of cannabis cultivation.

Cannabis is a short-day crop, benefitting from early- phase cultivation under long days to increase vegetative growth before transitioning to a generative flower phase. Typically, cannabis flowers under broad-spectrum lighting. Before LEDs became more prominent, its vegetative phase was grown under ceramic metal halide (CMH) lighting (due to its relative increased blue content) and flowering took place under high pressure sodium (HPS).

The increased blue light during young plant production is useful as it promotes plant compactness and root development. Ultimately, pre-flowering objectives include compact growth, thick stems and established root growth.

Cannabis is cultivated under light intensities three times greater than those used in leafy greens and ornamentals, so it may be possible to produce optimal young plant quality under relatively low blue content. Fluence tested this theory by utilizing 40% to 80% red light treatments on three cultivars – ‘Blue Dream,’ ‘Texas Original’ and ‘Tropickle’ – for 14 days, then moved them out for their flowering phase (an additional 56 days).

The results were not exactly earth-shattering. “Increasing red and decreasing blue did not affect plant height,” Poel reported. “There was no effect on plant height or flower weight or quality.”

What that means is it’s beneficial to cultivate cannabis plants during long days under a high-red spectrum to minimize lighting cost while avoiding any negative morphology effects.

But just because these results weren’t exciting doesn’t mean there’s not more work to do. Poel and Hawley said there’s additional ongoing work looking at extending highred into the plants’ short days. Another test will be increasing absolute levels of blue flux on pre-short day morphology.

“A lot of facilities run 24/7/365, so any improvement in growth could be very useful,” Poel said. “We’re looking at working with seedlings next for that early stretch period.”

by Courtney Llewellyn