Population density of industrial hemp (Cannabis sativa L.) in the field influences crop growth habit, fibre yield and quality. Therefore, optimization of plant population density is required to control growth and secure fibre yield and quality. Initial hand-thinned plant populations of 100, 200, 300 and 400 plants m-2 were established in a replicated field trial. Density of planting significantly influenced weed suppression and a number of phenological characters, and yield of industrial hemp. Weed suppression increased with increasing plant population. An increase from 100 to 200 plants m-2 markedly reduced weed weight from 23.2 to 6.5 g m-2. Further reductions in weed weights were observed at 300 plants m-2 (2.6 g m-2) and 400 plants m-2 (1.5 g m-2). Weekly height data showed that the high-density plantings resulted in shorter plants at harvest due to a more rapid decline in growth rate than for the low density planting (100 plants m-2). Stem thickness was inversely related to plant population density as low density produced thicker stems compared to that of high density planting. Leaf chlorophyll content and root mass m-2 were not significantly affected by differences in planting density. Raw fibre yields were greatest at 300 plants m-2, which was significantly higher in comparison to 100 plants m-2 (128.4 vs 102.8 g dry weight m-2). Yields were very poor overall with a maximum of 1.28 t ha-1 of raw bast compared to European yields of 2–3 t ha-1. Low yields were attributed to the unsuitable short photoperiods that caused early flowering and therefore shorter stem length in the current variety under trial. New varieties or crop management practices that delay flowering are necessary for regions of short day duration to produce economically viable fibre yields for the industry in subtropical Queensland, Australia.
Keywords: Industrial hemp, planting density, fibre yield, new crop