Until recently, most clonal cannabis has been propagated using fluorescent lights. Transitioning to LEDs may be a viable alternative to fluorescent lighting, enabling cultivators to provide specific spectrum treatments to enhance rooting while also saving energy. A recent study done by the University of Guelph compared a range of LED spectrum treatments, both fixed and temporally dynamic, with fluorescent lighting for the propagation stage of clonal cannabis. "The overall hypothesis is that some LED spectral combinations, either fixed or dynamic, will be superior to fluorescent lighting for producing cannabis transplants," the researchers said.
The study
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 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.
Effects on rooting and growth
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," the researchers conclude.
To read the complete study, go to journals.ashs.org
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"Light spectra have minimal effects on rooting and vegetative growth responses of clonal cannabis cuttings"
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