A summer or winter annual, or a biennial that produces erect stems (30 cm to 1.5 m in height) from a basal rosette of leaves. The leaf margin has fine prickles, and the midrib (lower leaf surface) is lined with conspicuous prickles. Reproduction is by seed.
Seedling: Cotyledons are rounded, tapering to the base, 7-8 mm long. Pale green leaves develop into a basal rosette. Young leaves are broadly club-shaped to egg-shaped, widest at the apex with distinctly toothed or spiny, wavy or slightly lobed margins. Stiff prickles are present on prominent midrib of lower leaf surface.
Mature Plant: Leaves are alternate (5-25 cm long), oblong to lanceolate, stiff and coarse, non-lobed or more often lobed with rounded sinuses. The leaf bases clasp the stem with ear-like lobes. The upper leaf surface is smooth; prickles are present on the leaf margin and on the midrib of the lower surface. Leaves often turn on edge and orient vertically toward the sun. Basal leaves are larger than upper leaves.
Leaves, roots and stems exude a milky juice when injured. Prominent midrib on under side of leaf has stiff prickles.
Flower Seed Head:
Flowering stems are erect and tall (up to 2 m). The main stalk of flowering stems is hollow. The panicle type seedhead branches widely. Numerous small yellow flowers are produced on the top portion of erect stalks. Flowers later turn white due to pappus on seed that allow for wind dispersal.
Nutlets are brown, oblong, and 3 to 3.3 mm long. Nutlets have five ribs on each face and a slender stalk (3 to 5 mm long) topped with white pappus.
Prickly lettuce is common in disturbed sites, gravel driveways, turf, and ornamental beds.