Tall Blue Lettuce grows as a tall, erect annual or biennial, reaching up to 12 feet high but usually around 7 feet. Stems are leafy, ridged, light green or reddish green, sometimes with purple streaks, contain a milky juice and are unbranched beneath the floral array. There can be sparse whitish hair.

Leaves vary considerably. The larger lower leaves tend to have deep pinnate lobes with coarse teeth on the lobes. The lowest have winged stalks, the mid-stem leaves touch the stem without a stalk. The smaller upper leaves are similar in shape and have two small lobes which partially clasp the stem. Most leaves are on the lower 2/3rds of the stem. The upper surface is a dark green and the lower surface is paler in color and the veins on the underside have whitish hairs.

The floral array is a number of long, narrow panicle arrays of small flowers formed on many side branches off the main stem.

The flowers are only 3/8 inch wide but have 20 to 30 small bisexual ray florets with light bluish, usually, or cream colored corollas and rays (seldom yellowish). Disc florets are absent. Only two species of Lactuca have florets that are usually bluish. The filaments of the 5 stamens and their anthers are appressed around the style which has a divided tip. Flower heads have involucres campanulate to cylindric in shape, longer than wide. Surrounding the outside of the head is a grouping of tightly appressed phyllaries with dark tips. These are usually reflexed when the seeds are visible.

The seed is mottled brown, flattened, ellipsoid shaped dry cypsela (seeds in composite plants that resemble an achene), with 4 to 6 nerve lines on the sides and a tawny-brown fluffy pappus attached for wind distribution.

Habitat: Tall Blue Lettuce grows in moist woods and thickets making due with partial sun. Even though this is a short lived plant, it develops a stout taproot.

Names: The genus Lactuca is applied to the lettuce family and comes from the Latin lac, for the milky juice of the stem and root. The species, biennis, refers to the plant being biennial (usually). The author names for the plant classification are: first to classify in 1794 assigning the name as Sonchus biennis was - ‘Moench’ which refers to Conrad Moench, (1744-1805), German botanist, Professor of Botany at Marburg, author of Methodus Plantas hirti botanici et agri Marburgensis, and who named the genus Echinacea. His work was amended in 1940 by ‘Fernald’ which refers to Merritt Lyndon Fernald (1873-1950) American botanist, Harvard Professor, scholar of taxonomy, author of over 850 papers, editor of the 7th & 8th editions of Gray’s Manual of Botany. In older literature several other names, which are no longer accepted, came into use between these two classifications - one was Lactuca spicata.

Comparisons: Similar in height and shape is the Wild Lettuce, L. canadensis. But there the florets are yellow in the majority of plants, the leaves are paler in color with finer teeth on the lobes. The only other species of Lactuca that typically has light blue ray flowers is Lactuca floridana, Florida Wild Lettuce, which a one time was also found in Minnesota but he DNR now lists it as historical. That species is less tall and has fewer ray flowers.