by Dr. Bob Blackshaw
Hemp-nettle (Galeopsis tetrahit) is a summer annual weed of the Mint family that has been present as localized populations in western Canada since the 1940s. It is most commonly found in the black soil zone but recently has been moving into wetter areas of the thin black and dark brown soils.
Hemp-nettle has square stems covered in bristly hairs and attains a height of 40-75 cm. Leaves are a pale green colour and occur as opposite pairs on the stem. Leaf blades are hairy, 5-10 cm long, and ovate-shaped with regular teeth on the margin. The pink to purplish flowers occur in whorls at leaf axils in the upper portion of the plant. Each flower produces four mottled gray-black seed with a prominent scar at the base.
Optimum seed germination occurs at warmer soil temperatures of 15-25 C and thus peak emergence usually occurs in mid-May through June. Seed is highly dormant and persists in the soil for many years.
Hemp-nettle has a deep taproot allowing it to consume considerable amounts of soil water and nutrients and thus it can be quite competitive with field crops. Infestation densities of 100-170 plants m-2 have been documented to cause 20-30% yield losses in barley, canola and wheat. The late spring emergence timing of hemp-nettle often allows it to escape a preseed glyphosate burndown treatment. Numerous herbicides exist for selective in-crop control but it is important to note that hemp-nettle populations resistant to Group 2 and Group 4 herbicides have been documented in western Canada. Careful herbicide selection is required depending on the prevalence of resistance in your area.