Flower:

Numerous small yellow flowers in an airy panicle at the top of the plant and arising from leaf axils in the upper part of the plant. Clusters hang in a classic shepard's hook prior to blooming. Flowers are lemon yellow, ¼ to ½ inch across, with 12 to 20 rays (petals).

The bracts (phyllaries) surrounding the base of a flower are variable in size, purple or green often with purple tips, hairless, overlapping, appressed, and the entire structure (involucre) forming a tube up to ~½ inch (9 to 12 mm) long. Flower stalks are green to reddish and hairless.

Leaves and stem:

Leaves are alternate, oblong to lance-oblong in outline, the lowest leaves largest, up to 12 inches (30 cm) long and 4 inches (10 cm) wide, deeply lobed, becoming smaller and often less lobed as they ascend the stem. Leaf edges have tiny teeth and short prickles all around. The leaf base has a pair of pointed lobes that clasp and usually extend past the stem.

Along the midrib on the underside is a line of stiff prickles. Leaf color is typically dark blue-green. Stems are erect, stiff, green to whitish or reddish, may have a few stiff bristles near the base but otherwise smooth, and usually many-branched on the upper stem. Leaves, stems and roots exude a milky white sap when broken.

Fruit:

Flower heads form seed heads about 1-inch in diameter with the bracts spreading out as seed matures. Seed is brown to yellowish or grayish, flattened, broadest near the tip and tapering at the base, with a stiff, slender beak 1 to 2 times as long as the seed body. At the tip of the beak is a tuft of white hairs to carry it off in the wind.

Notes:

A widespread weedy species of disturbed areas, Prickly Lettuce does not often encroach on high grade habitat but flourishes with the waste acreage provided by human activity. While there are several species with small yellow dandelion type flowers, Prickly Lettuce is most easily distinguished by the prickles on the leaves, and seeds that taper at the base and have a slender beak 1 to 2+ times as long as the seed body with white hairs at the tip.

Most similar is Canadian Wild Lettuce (Lactuca canadensis), which has pale orange-brown sap and flat, oval-elliptic seeds with a slender beak half to about as long as the seed body. Somewhat similar are the Sowthistles (Sonchus species), which also exude a milky sap, but have larger flowers and lack the stiff prickles on the leaf midvein.