Cannabis (Cannabis sativa) flourishes under high light intensities (LI); making it an expensive commodity to grow in controlled environments, despite its high market value. It is commonly believed that cannabis secondary metabolite levels may be enhanced both by increasing LI and exposure to ultraviolet radiation (UV). However, the sparse scientific evidence is insufficient to guide cultivators for optimizing their lighting protocols. We explored the effects of LI and UV exposure on yield and secondary metabolite composition of a high Δ9-tetrahydrocannabinol (THC) cannabis cultivar ‘Meridian’. Plants were grown under short day conditions for 45 days under average canopy photosynthetic photon flux densities (PPFD, 400–700 nm) of 600, 800, and 1,000 μmol m–2 s–1, provided by light emitting diodes (LEDs). Plants exposed to UV had PPFD of 600 μmol m–2 s–1 plus either (1) UVA; 50 μmol m–2 s–1 of UVA (315–400 nm) from 385 nm peak LEDs from 06:30 to 18:30 HR for 45 days or (2) UVA + UVB; a photon flux ratio of ≈1:1 of UVA and UVB (280–315 nm) from a fluorescent source at a photon flux density of 3.0 μmol m–2 s–1, provided daily from 13:30 to 18:30 HR during the last 20 days of the trial. All aboveground biomass metrics were 1.3–1.5 times higher in the highest vs. lowest PPFD treatments, except inflorescence dry weight – the most economically relevant parameter – which was 1.6 times higher. Plants in the highest vs. lowest PPFD treatment also allocated relatively more biomass to inflorescence tissues with a 7% higher harvest index. There were no UV treatment effects on aboveground biomass metrics. There were also no intensity or UV treatment effects on inflorescence cannabinoid concentrations. Sugar leaves (i.e., small leaves associated with inflorescences) of plants in the UVA + UVB treatment had ≈30% higher THC concentrations; however, UV did not have any effect on the total THC in thesefoliar tissues. Overall, high PPFD levels can substantially increase cannabis yield, but we found no commercially relevant benefits of adding UV to indoor cannabis production.

Until recently, most clonal cannabis (Cannabis sativa) has been propagated using fluorescent lights. Transitioning to light-emitting diodes (LEDs) may be a viable alternative to fluorescent lighting, enabling cultivators to provide specific spectrum treatments to enhance rooting while also saving energy. Vegetative stem cuttings of ‘Gelato-27’, ‘Grace’, and ‘Meridian’ were rooted for 15 days under various combinations of blue (B), red (R), ultraviolet-A (UVA) LEDs, phosphor-converted white (W) LEDs, and a fluorescent (F) control treatment, each with a canopy-level photosynthetic photon flux density (PPFD) of 200 µmol·m−2·s−1 and 16-hour photoperiod. The photon flux ratios of blue (B; 400–500 nm) and red (R; 600–700 nm) narrowband LED treatment combinations were (1) BR, fixed spectrum of B15:R85; (2) B, B75:R25 on day 0–2 followed by B15:R85 on day 2–14; (3) B+UVA, B75:R25 on day 0–2 followed by B15:R85 on day 2–14 plus 15 µmol·m−2·s−1 of UVA on day 7–14; (4) B50, B15:R85 on day 0–7 followed by B50:R50 on day 7–14. The W and F treatments both had static spectra. After the propagation period (i.e., plug stage), a portion of the cuttings under each treatment × cultivar combination were destructively harvested and the remainder were transplanted and grown vegetatively for an additional 21 days (i.e., transplant stage) under a PPFD of ≈275 µmol·m−2·s−1 from ceramic metal halide fixtures and then destructively harvested. Although there were no spectrum treatment effects on the percentage of cuttings that rooted, root index values were higher in cuttings grown under B+UVA vs. F. Further, relative root dry weights of plugs from the B, B+UVA, B50, and F treatments were higher than the W treatment. At the end of the plug stage, there were no spectrum treatment effects on the chlorophyll content index, cuttings grown under the B treatment had thicker stems compared with BR and W treatments, and cuttings grown under the F treatment exhibited the lowest percentage of new aboveground growth. None of the aforementioned spectrum treatment effects from the propagation stage persisted post-transplant. The use of LEDs is a promising, energy-efficient alternative to fluorescent lighting for cannabis propagation and B-enhanced spectrum treatments appear to enhance the rooting performance of clonal cannabis cuttings.