Abstract Hemp ( Cannabis sativa L .) was re‐legalized as an agricultural crop through the USDA 2018 Farm Bill, but cropping guidelines remain limited, especially for floral hemp grown for cannabinoid extraction. In this study, the response of direct‐seeded, field‐grown autoflower and full‐season hemp cultivars to varying rates of applied irrigation was compared over multiple years in a semi‐arid environment. Measurements included floral and total biomass, cannabinoid concentration, and cannabinoid yield. The primary flower size did not vary much regardless of irrigation amount, even for plants grown with no supplemental irrigation. As irrigation increased from very low levels up to 100% of the location's potential evapotranspiration (PEt) and beyond, leaf and stem biomass continued to increase as the plants also got taller. Secondary and tertiary flower biomass increased with irrigation, but only up to about 80% of PEt. Above that amount, floral biomass typically plateaued for both autoflower and full‐season types. Floral cannabinoid concentrations did not vary much regardless of irrigation amount. Irrigating at rates greater than about 80% of PEt does not generally increase cannabinoid yield. In contrast, under very dry conditions hemp can grow, reach maturity, and produce some flowers and associated cannabinoid yield even when receiving little or no rainfall or irrigation. Thus, growing hemp with low, moderate, or no irrigation in a semi‐arid climate may be economically feasible (despite the lower yield) in certain situations, depending on costs of irrigation water, other crop management decisions, and market valuation of end products.
Roseberg et al. (Wed,) studied this question.