persicaria, lady's thumb, willow weed
Persicaria maculosa Gray, (Polygonum persicaria L.)
Redshank is a native summer annual generally distributed throughout the UK on waste, cultivated and open ground. It is recorded up to 1,500 ft. It is a troublesome weed in cultivated fields and gardens. It occurs on most soils but especially on moist, rich soils and on acid peaty loams. It is said to thrive on soils deficient in lime. Redshank occurs in cereals and other arable crops but it is not associated with a particular crop and is regarded as a follower of man.
Redshank exhibits considerable phenotypic plasticity and genotypic variation. The leaves vary in shape and size and the flowers may be red, pink or white. It can form hybrids with pale persicaria (P. lapathifolia).
Redshank was regarded as nutritious and has been fed to horses and cattle as green food. The leaves are rich in vitamin C. However, the plant contains oxalates and is potentially toxic in large amounts.
Redshank flowers from May to September or until killed by frost. The flowers are sometimes pollinated by insects but are usually self-pollinated automatically. Seeds ripen from July onwards. Figures for the average seed numbers per plant vary between 200 and 5,952. The seeds are polymorphic and exhibit considerable variation both in weight and shape. Plants growing in the shade produce smaller seeds than those growing in full light. In dry conditions a plant will allocate more reserves to the seeds. Redshank can be found in fruit for 5 months of the year.
Seed is dormant at shedding and for 60 days afterwards. Chilling permits seed to after-ripen. Moist storage at low temperatures and seed scarification break seed dormancy. A period of dry storage is said to reduce seed viability but may increase germination levels initially. Light, nitrate and alternating temperatures interact to promote germination.
In the field, seedling emergence occurs from April to June with the main peak in April. Seed that does not germinate develops secondary dormancy with rising temperatures during late spring and summer. Low temperatures in the next autumn-winter period break dormancy again permitting germination the following spring.
In sandy loam soil, field seedlings emerged from the top 70 mm of soil with most seedlings from the surface 40 mm.
If the plant is cut back early it may persist into a second year.
Seeds can remain viable in soil for 45 years. After 10 years, there was up to 62% germination depending on depth of burial. After 30 years, germination was around 9%. Redshank seedlings were abundant on land ploughed for the first time after 45 years. Seeds broadcast onto the soil surface and then ploughed in had a mean annual decline rate of 24% and an estimated time to 95% decline of 10-14 years under cropping with winter or spring wheat. In dry storage, seeds gradually lose viability. There was a 50% reduction after 3 years and 100% loss after 7 years. Seed stored under granary conditions exhibited only trace viability after 20 years.
Ripe seed retained on the plant may contaminate cereal grain at crop harvest. Redshank seed has been found as an impurity in cereal, flax, grass and clover seed. The mean number of seeds found in red clover was 6 per 1,000 equivalent to 21,000 weed seeds per ha sown with the clover.
Seeds can pass unharmed through the digestive systems of horses, cattle and deer. Viable seeds have been found in cattle droppings. The seeds are also ingested and dispersed by birds. Redshank seed can float in water for 24 hrs and has been recovered from irrigation water.
Well-hoed root crops and surface cultivations in spring and early summer should keep it under control. However, in moist conditions, stem fragments can re-root allowing re-establishment after soil disturbance. Seeding should be prevented and care should be taken not to introduce seed with crop seed or in manure.
In spring wheat, increasing the sowing rate of the cereal and reducing the row width reduced redshank biomass and seed production. The narrow rows were mechanically weeded in June and July with a spring tine harrow.
Redshank seed is susceptible to soil solarization but the seedlings are relatively tolerant of ultraviolet-B radiation. Seeds in moist soil heated at 75 or 102°C lost viability after 12 hours and after 5 minutes respectively.
Updated November 2007.