corn sow-thistle, creeping sow-thistle, dindle, field milk-thistle
Sonchus arvensis L.
Perennial Broad-leaved Weeds
Perennial sow-thistle occurs widely on arable land and waste ground throughout the UK. It is native by stream sides, waysides on banks, dunes and shingle. It is recorded up to 1,500 ft in Britain. Perennial sow-thistle is considered to be of universal distribution on different soils. However, it prefers slightly alkaline to neutral soils and does not thrive in acid or highly alkaline soils. It grows better at higher levels of soil moisture. It can be a problem garden weed. In a survey of weeds in conventional cereals in central southern England in 1982, perennial sow-thistle was found in 1% of winter wheat, 2% of spring barley but not at all in winter barley.
An arable and a coastal ecotype have been distinguished. The arable form does not survive on the dry, nutrient poor dunes while the coastal ecotype cannot tolerate periodic soil disturbance. The ecotypes also differ in germination requirements.
Perennial sow-thistle is a host of several important crop pests and diseases. The plant is eaten by foraging animals including rabbits. The abundant latex suggests that the plant has some potential for hydrocarbon or oil production.
Flowering takes place from late July until early October. The flowers are insect pollinated and more or less self-sterile. Seeds produced by self-pollination are rarely viable. A few seeds become viable just 4 days after flowering. Full seed maturity is achieved after 10 days. Seeds are able to develop if the stems are cut down and left to dry 9 days after flowering. A few seeds may mature on stems cut down just 4 days after flowering. An average of 46 seeds are produced per flower head. Seed number per plant is around 13,000 but a single flower stem with many heads may have over 9,000 seeds.
Seed germination is comparatively rapid when day temperatures are high and night temperatures low. There is no absolute requirement for light but it does increase germination levels. In field studies, the main period of seedling emergence was March to May with a peak in April. However, when seed was sown soon after shedding in August around 80% had germinated within 26 days. The seeds appear to exhibit only a short period of dormancy and readily germinate when conditions are favourable. Most seeds germinate at 5 to 30 mm deep in soil. In favourable conditions a seedling may flower in its first year.
The aerial shoots do not survive the winter, the foliage and fine roots die in September and plants overwinter as buds on the thickened roots and the underground stems of aerial shoots. In the autumn, the roots develop a strong innate dormancy that is not broken by tillage. Shoot development from anywhere along the roots begins again the following spring. Stems begin to elongate in mid-June and flower buds are visible by the end of the month.
Persistence and Spread
Seeds may remain dormant in soil for at least 6 years. In cultivated soil seeds remain viable for up to 5 years.
Perennial sow-thistle spreads both by seed and by the creeping root system. Seeds are generally wind-borne and dispersal distances of 6-10 m have been recorded in light winds. Hooked cells at the tips of the pappus hairs enable the seeds to be carried on clothes and animal fur as a further aid to dispersal. Viable seeds have been recovered from irrigation water but seeds of perennial sow-thistle do not survive long when submerged. Seeds have been found as a contaminant in home-saved cereal seed. Seeds ingested by earthworms have been recovered intact and viable in worm cast soil.
The vegetative spread of perennial sow-thistle is due to the development of stems that arise from buds on the swollen roots. The roots are found mainly in the top 15 cm of soil and appear to live for at least 2 years. Radial extension of the roots is 2 to 3 m per year. Thickened roots that have reached 1-1.5 mm in diameter are able to regenerate when broken into fragments. Root segments less than 25 mm long with well-developed buds can produce new plants.
Perennial sow-thistle is susceptible to repeated tillage in the early part of the growing season. Fragmentation induces more buds on the root pieces to grow, using up food reserves. After defoliation, new aerial shoots come mainly as lateral shoots from the basal underground parts of the cut shoots. Repeated shoot removal over a period of 80 days is said to exhaust the underground organs. Shoot regeneration from buried root sections is much less around the 5-7 leaf stage, from late-May to early-June, when food reserves are naturally low. Regular defoliation of plants at the 4-6 leaf stage is reported to have killed plants within a season. Mechanical control is greater with increased fragmentation and deep burial in autumn. Ploughing in April followed by cultivations at 3-4 week intervals between June and September has given 99% control of perennial sow-thistle in the USA.
Laying badly infested land down to grass will crowd out the weed. Dense crops of lucerne, vetch or maize will also tend to shade it out. The inclusion of successive root crops in the rotation will help to destroy the weed. Winter cereals favour perennial sow-thistle, which has the winter, spring and summer to become established and enough time to flower and set seed before crop harvest.
Control should aim to prevent seeding as well as exhaust the rootstocks. Fallowing for one year reduced seed numbers in soil by 60%. Seed numbers remained low even after the land was cropped again.
Seeds of sow-thistle are often predated in the flower-head by beetle larvae. The plant is palatable to both cattle and sheep
Updated October 2007.