Medical Hemp Research
The crop physiology lab studies growth requirements, THC & CBD production, and other characteristics of hemp to provide further research for the use of the plant as a medical tool.
Cannabis lighting: efficacy is more important than spectra for yield and cannabinoids
F. Mitchell Westmoreland, Paul Kusuma & Bruce Bugbee
Crop Physiology Laboratory
Utah State University
Abstract for manuscript in review for Frontiers in Plant Science
High-pressure sodium (HPS) fixtures are commonly used in cannabis cultivation, especially during the flowering stage. HPS fixtures have a lower initial cost compared to light-emitting diode (LED) fixtures, but higher electric (operating) costs due to their lower efficacy. LED fixtures are often designed with unique spectra that have the potential to increase yield or cannabinoid profile (quality), but some spectra reduce fixture efficacy. We studied spectral effects on yield and quality of medical hemp in a walk-in growth room that had five photon-independent compartments and common atmospheric conditions. Four replicate studies were conducted. The photosynthetic photon flux density (PPFD; 400 to 700 nm) was identical among treatments in all studies. The fraction of blue photons ranged from 4% (HPS), to 20% (cool white LEDs). The UV photon flux was less than 1% of the total photon flux. The far-red photon flux was 7% of the total photon flux in the HPS and decreased to 2% in the cool white LEDs. Because HPS technology emits more thermal radiation than LEDs, leaf temperature is increased about 2 C and flower bud temperature by up to 4 C. In some studies, glass was mounted below an HPS fixture to make the plant temperature similar to the LEDs (at equal PPFD). Dry flower yield among studies ranged from 0.3 to 0.35 grams per mole of photons. There was a 1% increase in yield per percent decrease in blue photons, measured as either grams per mole or grams per m2. We used the medical hemp cultivar T1 (Trump), which tends to accumulate high levels of both CBD and THC. CBD at harvest was about 8% and THC was about 0.3%. The ratio of CBD to THC was about 25 to 1 in all treatments and all studies. There was no statistically significant effect of spectral quality on either THC or CBD concentration. As expected, plants grown under the low blue fraction of HPS were 15% taller than plants grown under LEDs. The efficacy of the fixtures ranged from 1.7 µmol per joule (HPS) to 2.5 µmol per joule (white+red LEDs). Because of the increased efficacy, the white+red LED fixture had 25% higher yield per dollar of electricity than plants grown under HPS, and 10% higher than LED fixtures with lower efficacy. These findings suggest that fixture efficacy and initial cost of the fixture is more important for return on investment than spectral distribution.