Virginia Tech Weed Identification Guide

Annual Sowthistle: Sonchus oleraceus

Weed Description:  An annual with bluish-green leaves and stems that secretes a milky sap when cut. Found throughout the United States.

Seedling: Cotyledons egg-shaped and have petioles. Young leaves are alternate, egg-shaped, and form a rosette. Both young leaves and cotyledons have a whitish coating.

Leaves:  Rosette leaves alternate, egg-shaped, with toothed margins. Stem leaves are alternate, without hairs, and have lobes that clasp the stem. Leaf margins are toothed and only slightly prickly.
Roots:  Taproot.

Stems:  Unbranched, 1-4 ft in height, and emit a milky sap when cut.

Flowers:  Several occur in clusters at the end of the stems, pale yellow, 1/4 to 3/4 inch wide.

Fruit:  An achene. Mature seed have a white feathery pappus that collectively form a white "puff ball" similar to dandelion.

Identifying Characteristics: The leaves with very prickly margins that initially develop as a basal rosette and then occur alternately along the flowering stem, the bluish-green color of the leaves, and the yellow flowers with a 'puff-ball' seedhead are all characteristics that help in the identification of annual sowthistle.  Spiny Sowthistle (Sonchus asper) is very similar in appearance, however this species has rounded lobes that clasp the stem whereas annual sowthistle has distinctly pointed lobes. Also, the leaf margins of spiny sowthistle are much more spiny or prickly than those of annual sowthistle.  Both of the sowthistles can be distinguished from 'true' thistles by the milky sap they emit when broken. The sowthistles might also be confused with Prickly Lettuce (Lactuca serriola), however the midvein on the leaf undersides of this weed has distinct spines or prickles, whereas those of the sowthistles do not.