Exploring the Legacy Practice of Flushing in Controlled-environment Production of High-CBD Cannabis (Cannabis sativa)

In controlled-environment cannabis ( Cannabis sativa L.) production, restricting fertilizer application and maintaining low substrate electrical conductivity (EC) before harvest, which is termed “flushing,” is a standard procedure that is hypothesized to reduce inflorescence mineral nutrient concentrations, thereby improving dried inflorescence smoking quality, without impacting inflorescence yield. The objective of this study was to determine if flushing impacts yield, concentration of cannabinoids, or mineral nutrient concentrations of inflorescences. Two high-cannabidiol (CBD) cannabis cultivars, THM Jack and Southern OG, were flowered under a 12-hour photoperiod harvested at 7 weeks or 8 weeks, respectively. ‘THM Jack’ was subjected to preharvest flush durations from 0 to 3 weeks, and ‘Southern OG’ was subjected to preharvest flush durations from 0 to 4 weeks. Inflorescence dry mass (kg·m −2 ) declined as flush duration increased, but trim dry mass was unaffected. Additionally, Cannabidiol (CBD) and tetrahydrocannabinol (THC) concentrations were not impacted in ‘THM Jack’, but CBD concentration increased with flush duration in ‘Southern OG’. The increase in CBD concentration did not compensate for the decline in inflorescence mass, which resulted in a decline in CBD yield (g·m −2 ). Nitrogen, phosphorus, and potassium concentrations declined in both cultivars as flush duration increased. Collectively, these results indicate that flushing can impact dry mass and tissue concentrations of cannabinoids and mineral nutrients. Whether these changes are beneficial depends on weighing a desired outcome (e.g., reduce fertilizer costs, improve smoking quality, manipulate cannabinoid concentration) against a loss in inflorescence and/or cannabinoid yield. Future research should evaluate how restriction of specific nutrients might impact inflorescence growth or organoleptic properties as well as potential differences in cultivar sensitivity to flushing based on seasonal variation in controlled environments.