Black strap, hogweed, iron weed, prostrate knotweed, surface twitch, red robin, wireweed
Polygonum aviculare L. ;(P. heterophyllum)
Annual Broad-leaved Weeds
Knotgrass is a procumbent annual weed frequently troublesome among cereals and root crops on most arable soils. It is common throughout Britain and is recorded up to 1,800 ft. Knotgrass is said to prefer soils deficient in lime. It is often abundant on light sandy soils and soils manured by sheep. Knotgrass also occurs on roadsides, waste ground and in gardens. While it is a plant of disturbed and trampled habitats, in urban situations it occurs in less trampled, paved areas. Knotgrass has some tolerance to salt.
Knotgrass is an important weed of cereals, sugar beet and other arable crops. It is common in the vegetation cover and in the soil seedbank. Knotgrass is present in the field margins too.
Flowering occurs from May to October. The flowers are probably self-pollinated. Average seed production by a large plant may be 6,380 seeds. Seed is produced from July onwards and knotgrass can be found in fruit for 4 months of the year. Seed rain from plants emerging after cultivation in April extended from August to November. Seeds are dormant when ripe and for 60 days after maturing. Scarification of seed and chilling for 8 weeks increases germination levels. Dormancy is relieved by low winter temperatures and re-imposed when temperatures rise in late spring.
Seedling emergence occurs from late February to June. In the field, most seedlings emerge from the surface 30 mm of a sandy loam soil with odd seedlings from as deep as 50-60 mm.
Persistence and Spread
Seed buried in soil gave 55% germination after 20 years and 4% after 50 years. Seeds broadcast onto the soil surface, ploughed to 20 cm and followed over a 6-year period of cropping with winter or spring wheat had a mean annual decline rate of 23%. The estimated time to 95% decline was 9 to 20 years. Other similar studies have suggested a mean decline per year of 60% and a time to 99% loss of 5 years. The life span of knotgrass seeds in dry-storage was less than 15 years.
Shed knotgrass seeds are taken by birds but invertebrates are more important post-dispersal seed predators at certain times of year.
Knotgrass seed was a contaminant of cereal, clover and grass seeds, particularly home-saved seed. Birds and mammals disperse knotgrass seeds. Seed has been found in cattle and horse droppings and in manure. Seedlings have been raised from the excreta of various birds. Seed has been recovered from irrigation water and was still viable after submergence in water for more than a year.
Unless seeding is prevented, eradication is impossible. The adoption of a rotation that includes more root or hoed crops may help to prevent seeding and aid control. The plant can regenerate if the top is cut off during the growing season.
A root crop such as kale, to be folded off by sheep, will almost exterminate the weed. Initially the seedbed preparations and early hoeing in spring kill many seedlings. Later the growing kale shades out the weed and, in folding, the close treading of the sheep prevents further weed growth before the land is ploughed.
In a comparison of tillage regimes in winter cereals, ploughing and other deep cultivation favoured knotgrass, while shallow cultivations discouraged it. In a wheat crop, 2 or 3 severe harrowings when the surface is crumbly will reduce the weed.
Two fallow crops in succession will help, particularly where surface cultivations in spring encourage germination for subsequent destruction by harrows. Fallowing for 1 year reduced seed numbers in soil by 75%, however, cropping with winter wheat for the same period reduced seed numbers to a similar extent with good weed control. Regular fallowing every 5th year over a 15-year period with a high level of weed control maintained in the intervening cropped years has reduced seed numbers to a low level.
Updated November 2007.