Perennial sowthistle Sonchus arvensis L.
Asteraceae (Aster family)
Erect, patch-forming perennial.
Seedling leaves initially develop from a basal rosette from a deep taproot. Lower leaves are alternate, blue-green, 4 to 12 inches long with two to five lobes with prickly teeth on each side, gradually becoming smaller towards the top with a clasping base. Leaves have a thickened midvein and exude a milky sap when damaged. Leaf midvein is not prickly underneath.
Erect, herbaceous stem up to 5 feet tall that branches only near the flowers. Stems are hollow and smooth with ridges and exude a milky sap when damaged. Upper stems may have gland-tipped hairs.
Flowers and fruit
Bright yellow ray flowers are 1 to 2 inches wide and found in clustered heads. The seed is enclosed in a single-seeded, narrow, oval, reddish brown, ribbed, wind-disseminated fruit.
Seeds and extensive creeping roots.
Annual sowthistle (S. oleraceus L.)
Differs by having an annual life cycle and short taproot, less prickly leaf margins, and pale yellow, 0.5- to 1-inch-wide flower heads. Leaf midvein is not prickly underneath.
Prickly lettuce (Lactuca serriola L.)
Differs by having an annual or biennial life cycle; fine prickles on leaf margins and sharp prickles on the underside of the leaf midvein; and a many branched, spreading panicle with numerous, pale yellow flower heads less than 0.5 inch wide.
Spiny sowthistle [S. asper (L.) Hill]
Differs by having an annual life cycle and short taproot, leaf margins with many sharp prickles, earlike lobes where the leaf attaches to the stem, and flower heads 0.5 to 1 inch wide. Leaf midvein is not prickly underneath.