Sonchus asper, Spiny-leaf Sowthistle
Leaves have round, spiny lobes that curve down and clasp the stem. The stem may be hairy or smooth.

Sonchus oleraceus, Delectable Sowthistle
Leaves have pointed, spiny lobes that clasp the stem. The stem is smooth.

Sonchus arvensis var. glabrescens, Smooth Field Sowthistle
Leaves clasp the stem, but the lobes are smaller than in the othe two species. The stem is hairy.

Sources: The Flora of Delaware, Herbaceous Plants of Maryland.