Prickly Sow-thistle

Unbranched, erect, reaching as much as 6 feet in height but generally around 2-3 feet in height. Stems emit a milky sap when broken.
Leaves initially develop as a basal rosette and for this reason are often confused with thistles. All of the leaves are deeply cut with two rows of lateral appendages (pinnatifid), and range from 2 1/2 to 12 inches in length and 1/2 to 6 inches in width. Leaves are generally hairless (glabrous), egg-shaped in outline and have prickly margins. Leaves that occur on the flowering stem are are alternate and have rounded lobes that clasp the stem. Leaves emit a milky sap.
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 spiny sowthistle. Annual Sowthistle (Sonchus oleraceus) is very similar in appearance, however this species has distinctly pointed lobes that clasp the stem whereas spiny sowthistle has rounded lobes. Also, the leaf margins of annual sowthistle are much less spiny or prickly than those of spiny sowthistle.