Effect of plant population on crop growth, floral biomass, and cannabinoid yield in field-grown hemp

Abstract

Hemp (Cannabis sativa L.) was recently re-legalized in the United States and cultivated for various purposes, including floral cannabidiol (CBD), grain, and fiber. Due to prohibition of hemp for a century, research supporting production guidelines, including plant density effects on CBD production, remains limited. This study determined the responses of field-grown full-season CBD hemp cultivars to plant stand densities ranging from 4000 to 86,000 plants ha⁻¹ from 2021 to 2023 in a semiarid, irrigated environment in Southern Oregon. Crop measurements included primary, secondary (2° colas) and tertiary floral (3° colas) biomass, Stem+Leaf biomass, cannabinoid concentration, and cannabinoid yield. Over an eightfold increase in stand density each year, plant morphology clearly changed. Stem+Leaf biomass per plant decreased by about fourfold, expressed as smaller plants having fewer side branches. The biomass of 2° colas decreased by about fivefold per plant, while 3° cola biomass also decreased to nearly zero at the highest densities. The primary flower size varied little with density. Since floral CBD concentration was almost constant at all stand densities, the CBD yield per hectare was controlled by the yield of floral material. While the per-plant biomass of floral material decreased as density increased, the simultaneous increase in plant number per hectare resulted in an apparent CBD “yield plateau” at a density of about 50,000–60,000 plant ha⁻¹ for Stella and about 70,000–80,000 plant ha⁻¹ for Lifter. These were 10–15 times the densities commonly seen in production fields in southern Oregon.