As a short-day crop, Cannabis sativa, benefits from early-phase cultivation under long days to increase vegetative growth before transitioning to a generative flower phase. Previously, this long-day or “veg” phase has occurred under ceramic metal-halide lamps due to their relative increased blue content compared to the spectra of high-pressure sodium lamps used during the short-day or “flower” phase. Increased relative blue light during young plant production is desired due to the promotion of plant compactness and root development. Due to the relative efficacy of red diodes to blue, there is a benefit to maximizing red content without affecting plant performance. While many spectral recommendations arise from research in leafy greens and ornamentals, cannabis is cultivated under intensities three-fold greater, 600 µmol·m-2·s-1 during the long-day phase to 200 µmol·m-2·s-1 in leafy greens. Therefore, due to the high light intensity, it may be possible to produce optimal young plant quality under relatively low blue content. To test this, three cannabis cultivars rooted for 14 days were transplanted into 2-gallon coco coir bags and grown under long days with spectral treatments for an additional 14 days before finishing in a 12-hour short-day common environment. During the long-day phase, plants were exposed to high (~80%) or low (~40%) red at an average intensity of 500 µmol·m-2·s-1. Plant height at transfer to short days was similar regardless of light treatment. Additionally, final plant height and total flower yield after transfer into short days in a common environment was also similar. Therefore, it is beneficial to cultivate cannabis plants during long days under a high-red spectrum to minimize lighting cost while avoiding any negative morphology effects.
