Winter annual, biennial, or sometimes annual.
Reproducing only by seed.
Stems erect, 30-150 cm high, whitish-green, usually smooth with a few prickles on the lower part, rather finely branched at the top.
Leaves variable in size and shape. Usually deeply lobed or nearly divided with backward-curving lobes but sometimes with irregularly shaped lobes or without lobes. The outer (convex) margin of each lobe usually weakly spiny-toothed, the inner (concave) margin usually without teeth or with much smaller teeth; leaves clasping the stem with basal lobes; underside of the midrib nearly always with a single row of stiff, sharp prickles (hence "prickly" in the common name), these usually absent from the upper leaves among the inflorescence and occasionally absent from the lower leaves of second growth after mowing; leaves alternate (1 per node), usually twisted near the stem so the leaf blade is oriented with the margins pointing vertically and the flat surfaces facing horizontally, leaf tips often (but not always) pointing north and south.
Flower heads small and very numerous on fine stalks in much-branched inflorescences; each head about 7-8 mm long and about 3 mm across, with 5 to 12 yellow ray florets, the yellow colour often fading to bluish on drying; disk florets absent; seeds narrowly oval with a long beak tipped with a tuft of white hair (pappus); whole plant with sticky white juice. Flowers from June to late autumn.
Prickly lettuce occurs throughout Ontario in waste places, pastures, roadsides, cultivated fields, and occasionally in gardens.
It is distinguished from most other plants having milky juice and lobed or prickly-margined leaves by the single row of firm, sharp prickles along the underside of the midribs of the stem leaves.
Prickly lettuce. A-D. Variations in lobing of leaves on different plants. E. Top of flowering branch.
2 leaf Prickly lettuce seedling.
6 leaf seedling plant.
Prickly lettuce leaf margins.
Undersip of leaf with the sharp prickles that exist along the midrib.
Flowering plant (approx. 75 cm tall).
Flower of Prickly lettuce.