Weed control in hemp farming
If hemp is planted in well-drained, fertile soil under nearly optimum temperature and moisture conditions, it will germinate quickly and reach 30 cm in 3-4 weeks from planting. At this stage it will give 90% ground shade. Weed growth is suppressed by the exclusion of light from the soil. It appears that rapidly growing hemp, at a final population of 200-250 plants/m2, will suppress nearly all weed growth, including twitch grass. For preplant site preparation guidelines, see OMAFRA Publication 75, Guide to Weed Control.
Weed suppression is not a permanent condition. Weeds may appear on the same field next year if the field is rotated out of hemp production. Perennial grasses may be weakened or killed if hemp is grown a second year on the same ground. However, this practice increases the opportunity for crop diseases to develop.
Under grain production conditions, weed suppression may be less complete. The lower plant population or uneven stands allow more light to penetrate the canopy, aiding the germination of weed seeds. Cross seeding may improve canopy distribution and subsequent weed control where very early, shorter varieties are grown.
Early planting, as soon as the soil is warm enough, is a recommended weed control strategy.