What impact does supplemental light have on hemp production?

During 2020 New York greenhouse growers were estimated to produce over 9 million square feet of industrial hemp reports Hemp Industry Daily. Cornell University horticulture professor Neil Mattson, who is a member of the Cornell Hemp Research Team, is conducting a variety of studies related to greenhouse hemp production including the use of supplemental lighting.
“The New York State Industrial Hemp Agricultural Research Pilot Program is what initially got us started with our hemp lighting research,” Mattson said. “We have also developed some broader hemp objectives related to light, fertilization and substrates.”

Last winter Mattson conducted a study with the hemp strain ‘TJ’s CBD’, which according TJ’s Gardens tests from 13-18 percent cannabidiol (CBD). Plants were grown under eight different light treatments, including high pressure sodium (HPS), metal halide and LED lights. Six different LED treatments were trialed including a white broad spectrum LED, four treatments consisting of different ratios of red and blue light and one treatment where plants were grown under HPS lights and then finished under high blue light.

Placement of the light fixtures in the greenhouse was adjusted so that the light output delivered to the plants was the same for all treatments (i.e. 200 μmol·m-2·s-1 at the center of each treatment, as measured 3 feet above bench height).

“In terms of light quality, the main significant differences we found a slightly higher yield, measured as the flower dry weight, for white LEDs as compared to the higher 70 percent red/30 percent blue (70:30 R:B) treatment and 80:20 R:B treatment as well as for the metal halide fixtures,” Mattson said. “The high pressure sodium light treatment, which was the control, the 90:10 R:B treatment and 60:40 R:B treatment, weren’t significantly different than any of the other light treatments so they statistically performed as well as the white LEDs.”

This winter Mattson will be replicating the lighting study, but will reduce the number of light treatments.
“Because there weren’t significant differences between the red/blue LED treatments, we are reducing the number of LED treatments,” he said. “We are focusing on the yield differences between white LEDs, high pressure sodium and a couple different red/blue treatments.”

Differences in CBD and THC levels

In addition to looking at differences in flower dry weight between the light treatments, Mattson did an analysis of the percent CBD and tetrahydrocannabinol (THC) in the flowers. He found there weren’t any significant statistical differences in the CBD level based on the different light quality treatments.

“Individual plants seemed to vary from 8-12 percent CBD,” he said. “Those values didn’t seem to be correlated to the lighting treatment. For many CBD strains, 10 percent would be considered a good CBD level. A grower can try pushing the CBD level higher, but then there is the concern about increasing the THC level above the legal limit for medical cannabis.”

The THC level for the flowers produced under the different light treatments averaged 0.47 percent.

“Like CBD, the THC level did not seem to respond to light treatments,” Mattson said. “The cut-off for THC level to be legal medical hemp has to be less than 0.3 percent (0.39 percent to allow for different lab reports to have 0.1 percent accuracy). For the ‘TJ’s CBD’ plants that we grew, the average THC level was 0.47 percent so it was considered to be “hot.” Of the flower samples that we collected 90 percent were considered hot with the THC level above 0.39 percent.

“The THC level produced in our trials has implications as to which week plants should be harvested. The longer a grower waits to harvest the plants typically CBD levels increase, but THC levels can also increase. I expect the flowers in our trials should have been harvested one week earlier so CBD levels would have been 8-10 percent and the THC level would have been below 0.3 percent.”

Larry Smart, who is the Cornell University Hemp Research Team project lead, has done studies with field-grown CBD hemp strains harvesting hemp plants at different times at one week intervals.

“I have seen some data which showed CBD and THC levels increased as the time to harvest field-grown plants was delayed,” Mattson said. “There can be other stresses under field conditions that don’t occur in a greenhouse or indoor grow. These other field stresses could cause the plants to go hot.

“Medical and recreational cannabis are the same species of plant as hemp cannabis so there could be some biological basis that our results would extend to THC strains. Some strains respond differently than others, so there is some basis that the results we found for the CBD strain trialed would extend to THC strains. This winter we will be adding two more CBD strains, ‘T2’ and ‘Janet’s G’. This will give us a better indication as to how applicable our results are from strain to strain.”

Determining profitability of greenhouse hemp crop

Mattson said he still has questions about the economics of growing CBD hemp in a greenhouse other than propagating young transplants for field production.

“Some of our work has showed there are valid quality control issues that can be controlled in a greenhouse that can’t be controlled in the field,” he said. “We have heard horror stories about field hemp growers who were bringing a crop into harvest and then experienced a lot of rain or an early frost. In the case of the excessive rain, plants experienced outbreaks of gray mold (Botrytis), powdery mildew or other disease issues. These growers produced good crops all summer and then lost them because outdoor conditions made it difficult to harvest the plants.”

Mattson is still working on determining the economic feasibility of growing finished hemp crops in greenhouses.

“There are a number of factors involved with determining the profitability of growing a finished greenhouse hemp crop,” he said. “Is it possible to produce a higher yield in a greenhouse using a higher plant density? A greenhouse can yield several crop turns per year vs. one field-grown crop. Also, can a greenhouse produce higher quality products? All of these factors could contribute to the economic benefits of greenhouse production compared to field production.”

GLASE research expands to include hemp

In addition to conducting research as a member of the Cornell Hemp Research Team, Mattson has begun to conduct hemp lighting studies as a principal investigator for the Greenhouse Lighting and Systems Engineering (GLASE) consortium.

“With funding from GLASE we will expand the greenhouse lighting research that was funded by the state hemp project looking at the effect of light quality on CBD hemp yield and CBD content comparing LEDs and HID lighting,” Mattson said. “With the GLASE research we will conduct more in-depth studies on the energy efficiency of the different lighting sources as well. For example, the research could show that the flower dry weights and CBD yields are roughly the same for LEDs and HID, but the electrical consumption for the LEDs could be found to be 40 percent lower.”

Another objective of the GLASE hemp research is to study light quantity or daily light integral on hemp production. Mattson grew three autoflowering strains (‘AutoCBD’, ‘Maverick’ and ‘Pipeline’) at four different DLI levels (15, 20, 25, and 30 mol·m-2·d-1) to determine the effect on shoot and flower dry weights and CBD levels. Analysis of flower weight and CBD is still ongoing, but initial results on total plant biomass have been determined.

“One of the things we found is how much light cannabis plants need to receive,” Mattson said. “Cannabis growers talk about plants needing a minimum DLI of 30 mol·m-2·d-1. For the strains ‘AutoCBD’ and ‘Pipeline’, we didn’t see any increase in the plant shoot dry weight above 20 moles of light. For the strain ‘Maverick’ there was a linear increase in shoot dry weight as the DLI increased from 15 to 30 moles. The flower analysis still needs to be completed before we draw any concrete conclusions.”

Mattson is also planning to study the potential benefits of carbon dioxide enrichment with hemp plants.

“Greenhouse vegetable growers are aware of the benefits of supplementing with carbon dioxide,” he said. “We will be doing studies with hemp to determine the photosynthetic response to supplemental light and carbon dioxide to increase yields. This will provide us with insight into the interaction between light and carbon dioxide and the photosynthetic benefits of carbon dioxide supplementation to boost yield or reduce supplemental lighting.

“A second phase of the project will compare the yields using supplemental light alone vs. a combination of supplemental light and carbon dioxide. We will be measuring the electricity savings related to using both supplemental light and supplemental carbon dioxide.”

For more: Neil Mattson, Cornell University, School of Integrative Plant Science, Horticulture Section, Ithaca, NY 14853; (607) 255-0621; nsm47@cornell.edu; https://hemp.cals.cornell.edu/; http://www.greenhouse.cornell.edu.