Effect of supplemental under-canopy lighting on cannabis.
You may have heard about under-canopy before, or maybe even saw a grower with light bars below the plants and wondered if it is beneficial in any way. Do you think it’s a bad or crazy idea to add additional lighting from below, as well as above?
Due to the novelty in LEDs and under canopy light studies, we receive a lot of questions and messages about the effectiveness of lighting plants from below. “Plants won’t absorb the light, worse – will bend, and energy will be wasted?”
Follow our grow journey and you will see some very sure comments dismissing the idea altogether. The results of our grows allow us to state with confidence that the increase in yield, uniformity, and increase in terpenes is no joke.
When creating the Sea of Beam, we started from real scientific research on the influence of supplemental under-canopy lighting on cannabis plants, conducted by the University of Guelph. In this article, we want to summarise the results of this experiment.


In a flower room, four benches with dimensions 6’x12’ were each populated with 35 plants for a total of 140 plants (figure A). The intensity of 500 umols/m2 /sec to the top of the canopy was provided by 315W ceramic metal halide top light above each bench. Under-canopy supplemental lighting added 95 ±5 umols/m2 /sec below the canopy.
Figure. A. Experimental Setup
Figure. A. Experimental Setup
The plants were exposed to three different light treatments below the canopy – Red+Blue, Full Spectrum, and No Light.
Column 1 had a red + blue (R+B) LED light bar (pink color in figure A), column 3 was exposed to a Full Spectrum LED light bar (green color in figure A), and column 5 was the control group which had no under-canopy light. (black color in figure A). Plants with an “X” were not included in the analysis because they were exposed to two different spectra at the same time.



Bud Yield

The addition of under-canopy lighting significantly increased dry bud yield as compared to the control group that did not receive supplemental light. The R+B under canopy LED light increased dry bud mass by 19.8%, while the Full Spectrum LED light increased yield by 24.5% – see figure B.
Figure B. Dry Bud Mass


Bud to non-bud tissue ratio

Figure C shows both R+B and Full Spectrum under canopy treatments significantly increased the ratio of bud to non-bud tissue.
Figure C. Bud to non-bud ratio


THC Cannabinoid Content

Under-canopy lighting provided a local impact on the production of Δ9-Tetrahydrocannabinol (THC). As shown in the figure below, lower canopy concentrations of Δ9-THC were increased when the plants were illuminated with under-canopy supplemental lighting treatments.
Figure D. THC concentration
A comparison of THC concentration between the top of the canopy and the lower canopy is shown in figure E. The figure shows that under-canopy lighting not only increased THC concentrations in the lower canopy, but in the upper canopy as well.
Figure E. THC Concentration, Lower VS Upper canopies.


Terpene Content

Supplemental under canopy lighting significantly increased the concentrations of select terpenes including alpha pinene, borneol, and nerolidol in both the lower and upper canopies – see figures F & G.
Figure F. Terpene concentration in upper canopy
Figure F. Terpene concentration in lower canopy




Results from the investigative study conducted at The University of Guelph indicate that supplemental under canopy lighting does increase bud yield, bud to non-bud tissue ratio, THC concentration, and selected terpene concentrations. Beneficial increases were observed both in the upper and lower canopies.
Improving Cannabis Bud Quality and Yield with Subcanopy Lighting, University of Guelph, 2018


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