LED spectra and defoliation independently shape canopy architecture and cannabinoid yield in indoor Cannabis cultivation

Cannabis growers often use light spectra and defoliation regimes to alter the growth of their plants. While several studies consider the impact of these practices separately, there are no reports of how these factors interact. This study used a fully factorial design to examine the individual and combined effects of LED light spectra and defoliation on plant architecture and cannabinoid production in two low tetrahydrocannabinol (THC) cultivars of Cannabis sativa grown indoors. The primary objective was to identify cultivation strategies that optimise canopy structure for labour efficiency while maximising cannabinoid yield. Comprehensive phenotyping revealed that both light spectra and defoliation significantly influenced plant growth, morphology, and cannabinoid profiles, with responses varying by cultivar but with minimal interaction between the two factors. Red-white (RW) spectra promoted vertical growth and altered canopy architecture, while blue-white (BW) spectra produced compact plants with increased cannabinoid concentrations but reduced floral biomass. The blue-red-white (BRW) spectrum provided an optimal balance of manageable plant architecture and high cannabinoid yield, making it especially suitable for space-limited indoor cultivation. Defoliation reduced plant height and floral biomass but increased cannabinoid concentrations in Black Label (BL), with more variable effects in Mountain Strong CBD 1 (MSD1). Notably, a negative correlation between inflorescence weight and cannabinoid concentration was observed, indicating a resource allocation trade-off. These findings underscore the importance of tailoring light and canopy management strategies to specific cultivars and production goals. • Effect of Light Emitting Diodes spectra and defoliation on Cannabis sativa. • Red-White LEDs promote vertical growth; Blue-White LEDs yield small, potent plants. • Blue-Red-White LEDs balance manageable structure and high cannabinoid yield. • Defoliation boosts cannabinoids in Black Label, but not always for Mountain Strong CBD1. • Growers must tailor light and pruning regimes to cultivar and production goals.