Flower:

Branching clusters of stalked flowers at the top of the plant and arising from upper leaf axils. Clusters are initially tightly packed and spread out some as the plant matures, but flowers generally remain fairly crowded, the cluster longer than wide. Flowers are ¼ to 3/8 inch across with 15 to 30+ pale blue to whitish rays (petals), rarely yellow. The bracts (phyllaries) surrounding the base of a flower are variable in size, green often with purple tips, hairless, overlapping, appressed, and the entire structure (involucre) forming a tube about ½ inch (9 to 14 mm) long. Flower stalks are green and hairless.

Leaves and stem:

Leaves are alternate and rather variable. Those near the base of the plant are up to 15 inches (to 40 cm) long and 8 inches (20 cm) wide, deeply divided into several wide, somewhat angular lobes with pointed tips, roughly triangular or arrow-head shaped in outline, with “winged” stalks. Leaves become smaller with fewer lobes as they ascend the stem; uppermost leaves are more lance-elliptic and may be stalkless and unlobed.

The upper surface is hairless to sparsely hairy, the lower typically has short sparse hairs along the veins, especially the midrib. Edges are often coarsely toothed but may be toothless or nearly so.

Stems are hairless to sparsely hairy, green, purple or purple streaked, and unbranched except in the flower clusters. Leaves, stems and roots exude a milky white sap when broken.

Fruit:

Flower heads form seed heads about 1-inch in diameter with the phyllaries spreading out as seed matures. Seed is 4 to 5.5 mm long, brown, often mottled, flattened, generally elliptic, sometimes the tip extending to a stout beak up to 1 mm long. At the tip of the seed/beak is a tuft of light brown to grayish hairs to carry it off in the wind.

Notes:

Tall Blue Lettuce is a common species that can be found in a variety of habitats, but most often in woods, wetlands and wetland edges in part to full shade. It is recognized by the small pale blue to whitish flowers, each with 15 to 30+ rays, that are in a fairly crowded branching cluster; leaves hairy on the veins and typically with toothy lobes that are somewhat angular in shape; seeds have a short (or absent) stout beak and have light brown to grayish hairs at the tip.

Most similar are Canada Wild Lettuce (Lactuca canadensis) and Woodland Lettuce (L. floridana), both of which can also be found in woods and may grow side-by-side with L. biennis. L. canadensis has pale orange-brown sap, yellow to pinkish or orangish flowers, seeds are smaller, flattened oval-elliptic with white hairs at the tip of a thread-like beak half or more as long as the seed body, leaves are often clasping with basal lobes, and may have hairs only on the midrib. L. floridana flower clusters are more open and diffuse, the flowers typically with only 10 to 15 rays; leaves typically have a large triangular lobe at the tip and a long, narrowly winged base; seeds have white hairs at the tip.