Sow Thistle

Sonchus oleraceus

Sow Thistle is a winter annual. It reaches 6 to 24 inches in height and has spiny lobed leaves. The flower head is made of numerous yellow ray flowers and no disk flowers and is 5/8 to 1 inch in diameter. Sow thistle is considered a weed in most yards and usually blooms all year. The milky sap of this plant was once used for eye ailments and skin imperfections. The leaves are eaten by white-tailed deer.

Sow thistle is a weak, succulent annual that forms a coarse, leafy rosette. Although somewhat similar to the thistly family, it is not a true member. Basal or rosette leaves of sow thistle are deeply lobed and have soft prickly spines along the margins. Upper leaves are not lobed and clasp the main stem. Yellow flowers are formed in clusters at the top of the plant. Each flower, composed of many tiny ray flowers, bears a striking resemblance to those of dandelion.

Sow thistle thrives in nursery crops, orchards, disturbed areas resulting from overgrazing, natural processes or cultivation. This weed may compete with other desirable plant species. The seeds are easily disseminated by the wind due to the pappus of white hair at one end.

The over wintering rosette phase may also serve as a reservoir for both vectors and viruses.This weed can indirectly affect the development of field crops because it hosts nematodes, aphids, and viruses.

Regular cultivation will stimulate germination. Tillage can be used for control at the emergence phase but may in turn allow for multiple flushes. Any rainfall event can stimulate the germination of annual sowthistle.

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 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.