Spiny annual sow-thistle, spiny-leaved sow-thistle, spiny milk-thistle, spiny sow-thistle
Sonchus asper L.
Annual Broad-leaved Weeds
Prickly sow-thistle is a troublesome annual or overwintering weed common on arable land. It is also found in gardens, on roadsides, waste places and other disturbed habitats. It is recorded up to 1,500 ft in the UK. Prickly sow-thistle grows on most soils but prefers well drained slightly acid to alkaline soils, and has some tolerance of saline conditions.
In a study of seedbanks in some arable soils in the English midlands sampled in 1972-1973, prickly sow-thistle was recorded in 53% of the fields sampled in Oxfordshire and in Warwickshire. Prickly sow-thistle seed was found in less than 1.5% of arable soils in a seedbank survey in Scotland in 1972-1978. In seedbank studies in arable fields in France, prickly sow-thistle was well represented in the seedbank and in the emerged vegetation.
A number of ecotypes and varieties of prickly sow-thistle are described but environmental conditions also influence the appearance of the plant. It is generally less abundant than the smooth sow-thistle, S. oleraceus.
Prickly sow-thistle has been used as a potherb since ancient times. It is host to various aphids and acts as a reservoir to several important plant viruses including beet western yellows.
Prickly sow-thistle flowers from June to October. The flowers are self-compatible. Mature seeds (achenes) are formed 1 week after flowering. The average number of seeds per flower head is 198, and a plant often has over 100 flower heads. Seed numbers per plant generally range from 21,500 to 25,000 but a large plant may have 60,000. In a competitive cereal crop a plant may have just 500 seeds. Moisture stress also reduces the number of seeds formed. Prickly sow-thistle can be found in fruit for 3 months of the year.
Seeds germinate from spring to autumn. Light and stratification at low temperatures stimulate germination. Seeds on the soil surface germinate better than those buried at 30 mm deep. Seedling emergence occurs from March to November, with peaks in March-April and August to November, but odd seedlings can germinate at any time. Spring emerging seedlings reach the rosette stage after 6 weeks. This is followed by flower bud formation and stem elongation.
Persistence and Spread
The half-life of seeds in cultivated soil is just 1 year while in dry storage it is 2-3 years.
The plumed seeds are normally wind dispersed and seeds have been collected by aircraft at 2,000 ft. Under damp conditions the pappus of hairs collapses and dispersal is prevented. The seeds are eaten by birds and viable seeds may be found in their droppings. Viable seeds have also been found in cow manure. Prickly sow-thistle seeds ingested by earthworms have been found intact in the worm casts. The seeds have been recovered from irrigation water. Prickly sow-thistle seeds have been found as a contaminant in clover, grass and cereal seeds, particularly in home saved cereal seed.
Seeding should be prevented by cutting, pulling or hoeing the plants before flowering. However, plants that are cut down early in the year can produce further flower stalks. In pasture, prickly sow-thistle may be controlled by grazing with sheep or mowing.
Tillage and surface cultivations in spring and the inclusion of root crops in the rotation should keep the weed in check. Regular cultivations will help to deplete the seedbank.
Direct heating of prickly sow-thistle seeds in soil reduces viability but high temperatures are needed to kill seeds relatively quickly. The seeds are susceptible to soil solarization.
Prickly sow-thistle is attacked by a range of insects.
Updated October 2007.