Flower:

Many small, loose open clusters (corymbiform) of yellow dandelion type flower heads, each up to 1 inch across when fully open, at the top of the stem and arising from leaf axils. The flower head, consisting of bracts, receptacle and ray flowers (petals), is thick, barrel to vase shaped, usually with tiny glandular hairs on the stalk just below the cluster. Bracts are in 3 or 4 layers, those in the outer layer shorter and broader at the base, sometimes with sparse glandular hairs, those in the inner-most layer long and narrow. Several small attending leaves are typically at the base of cluster.

Leaves and stem:

Upper leaves are up to 6 inches long with a rounded, widely lobed (auriculate) base semi-clasping the stem. Lower leaves are up to 10 inches long and 3½ inches wide, more spatula shaped with deeper lobes. All leaves are coarsely toothed with soft spines and are shiny waxy green on the upper surface. Stems are lined or grooved, green or reddish, usually hairless, stout, hollow, crisp but brittle, and exude a milky sap when broken.

Fruit:

Seed heads finely fuzzy with bracts fully open below. The brown seed is flattened and generally oval with 3 or 4 ribs per side, and a tuft of white hairs.

Notes:

Spiny Sowthistle has become widely cosmopolitan throughout the Americas and Asia due to human activity. A prolific and persistent weed of unattended gardens, it does not encroach readily into high grade habitats and is likely highly under-reported within Minnesota. It is distinguished from other sowthistles mostly by the auricled, prickly-toothed leaves. It is also more prickly than Common Sowthistle (Sonchus oleraceus) and has smaller flowers than Perennial Sowthistle (Sonchus arvensis).