Annual sow-thistle, milk-thistle, swine thistle
Sonchus oleraceus L.
Annual Broad-leaved Weeds
Smooth sow-thistle is a native annual or overwintering weed common on arable land on most soils throughout the UK. It is also found on wasteland, roadsides and in gardens. Smooth sow-thistle is a pioneer species that colonises disturbed ground but it prefers nutrient rich soils. Bare soil exposed by rabbits, burning or human activity offers favourable conditions for smooth sow-thistle emergence. Smooth sow-thistle is recorded up to 1,250 ft in the UK. It has a broad tolerance of climatic variation.
Smooth sow-thistle exhibits considerable variation in leaf form and flower colour. A number of ecotypes and varieties are described. Light levels also affect the appearance of the plant. In shady hedgerows the plants are green, in open areas they have a reddish hue.
It has been used as a potherb and as a herbal remedy for fevers and high blood pressure.
The plant is host to several aphid species and nematodes. It acts as a reservoir for some important plant viruses such as beet western yellows.
Smooth sow-thistle flowers mainly from June to August but flowering can start in April and continue until the first frosts. The flowers are self-fertile. Mature seeds are formed 1 week after flowering. Plant stems cut in bud do not ripen viable seed but the seed from plants cut in flower may be 100% viable. The average seed number per flower head is 140 and the number of flower heads per plant is around 44. The potential seed number per plant varies considerably with environmental conditions and estimates range from 5,000 to 40,000. Smooth sow-thistle can be found in fruit for 5 months of the year.
Seedlings emerge from April to September but the main flush occurs from April to June. Seeds collected and sown the same day will germinate mainly in the month after sowing. Under some conditions seeds require light for germination and this may restrict emergence in taller vegetation. The base temperature for germination ranges from 5.3 to 6.8°C. Seed germinates best on the soil surface or in the upper 20 mm layer.
In short days the seedlings develop a rosette of leaves. Elongation and flower stem development occur in long days. Winter rosettes can withstand a moderate frost.
Persistence and Spread
Seed recovered from excavations and dated at 150 years old has been reported to germinate. However, the half-life of seed in cultivated soil may be just 1 year. Dry-stored seed remains viable for around 10 years.
The seed has a pappus of hair and is wind dispersed in dry conditions. Tests suggest maximum seed dispersal distances of 4.4 and 6.6 metres at wind speeds of 10.9 and 16.4 km/hour respectively but this would be affected by release height. In wet conditions the pappus collapses and dispersal is limited.
Smooth sow-thistle seeds have occurred as a contaminant in clover, grass and cereal seeds. The seeds form a part of the diet of several birds and seedlings have been found to emerge from their droppings. The seeds have been recovered from irrigation water and can float in water for several days.
Seeding should be prevented by hoeing or by hand pulling. Topping plants to prevent seeding may result in the production of further flower stems. Surface cultivations during the main emergence period in spring and the inclusion of root crops should help to keep the weed in check. Regular cultivations will help to reduce seed numbers in the soil seedbank. Smooth sow-thistle is favoured by non-tillage systems.
Smooth sow-thistle does not survive beyond the seedling stage if it is shaded to any great extent. It cannot withstand repeated trampling and may be controlled by sheep grazing or mowing. It is also browsed by rabbits and hares. The plant is attacked by a range of insects, fungi and bacteria.
Smooth sow-thistle seed is susceptible to solarization.
Updated October 2007.