bee-nettle, blind-nettle, day-nettle, dog-nettle, glidewort, holyrope
Galeopsis tetrahit L.
Common hemp-nettle is an annual weed sometimes plentiful on arable land, and in cornfields. It also occurs in damp places and woodland clearings and is common throughout UK. It prefers nutrient rich loamy or sandy soils. Dry conditions reduce vegetative and reproductive growth. In an early survey of Bedfordshire and Norfolk it was characteristic of lighter soils and rare on chalk.
Common hemp-nettle seed was found in 9% of arable soils in a seedbank survey in Scotland in 1972-1978. In Narcissus crops common hemp-nettle reduced bulb yield due to its shading effect.
Common hemp-nettle is a very variable species in flower colour and leaf shape. It is thought to have originated as a hybrid between G. speciosa and G. pubescens.
The plant has some medicinal and therapeutic uses. The seeds are said to be included in the diet of marsh and willow tits.
Common hemp-nettle flowers from July to September. The flowers are self-fertile and mainly autogamous. Seed is set from August onwards. Flowers near the top of the plant produce 4 mature seeds while those lower down may have just a single seed. Seed numbers per plant can reach 2,800 but the average is 387. In cereals the average seed number per plant ranges from 34 to 68, and in root crops from 580 to 617. The seeds are shed as they mature.
Seeds from different populations show considerable variation in dormancy levels. Removal of the seed coat promotes germination in dormant seed. In field soil, most seeds germinate between 5 and 40 mm deep. The first seedlings emerge in early spring with further flushes of emergence throughout the growing season. There may be a late flush of emergence in the autumn. Common hemp-nettle seed mixed with soil and stirred periodically emerged mainly from March to May.
Common hemp-nettle seeds broadcast onto the soil surface, ploughed in and cropped with winter or spring wheat over a 6-year period had a mean annual decline rate of 61% and an estimated time to 95% decline of 3-5 years. Seed buried in soil in sub-arctic conditions had 46% viability after 2.7 years but was not viable after 6.7 years. Common hemp-nettle seed recovered from excavations and dated at 25-30 years old is reported to have germinated.
Seed shed at maturity may be dispersed by wind and water but there is no obvious mechanism for dispersal. Seeds may float in water for 2 days. Viable seed has also been found in cattle droppings.
Common hemp-nettle seed was a common contaminant in cereal seed samples tested between 1961 and 1968, and 1978 and 1981. In a survey of cereal seed in drills ready for sowing on farm in spring 1970, common hemp-nettle seed was found in 6% of samples tested. Most of these were from home saved seed. Contamination was much greater in pre-certification samples of barley seed than in certificated seed samples.
Control is by surface cultivations in spring and autumn. Regular hoeing in spring will largely destroy the seedlings. Delayed crop sowing in spring allows the first flush of weed seedlings to be killed before drilling or crop emergence.
Common hemp-nettle causes yield losses in spring barley. Relative time of crop and weed emergence is a major factor in determining the severity of loss.
Updated November 2007.