A Perennial herbaceous plant with a single stem and branches near the top into several flower stalks. Broken stems emit a sticky milky bitter juice with a sour odor. Sow Thistle alternate, lower leaves are deeply lobed, upper leaves clasp the stem; similar to dandelion leaves except with teeth ending in small weak prickles. With bright yellow flowers up to 2" wide daisies, blooming from June through August. The widely spreading white brittle roots penetrate five to ten feet, producing new plants from small root pieces.

Mature sow thistle stems can range from 30 cm to 2 m (1 to 6 feet) tall, depending upon species and growing conditions. Colouration ranges from green to purple in older plants. Sow thistles exude a milky latex when any part of the plant is cut or damaged, and it is from this fact that the plants obtained the common name, "sow thistle", as they were fed to lactating sows in the belief that milk production would increase. Sow thistles are known as "milk thistles" in some regions, although true milk thistles belong to the genus Silybum.

Sow thistles have been used as fodder, particularly for rabbits, hence the other common names of "hare thistle" or "hare lettuce". They are also edible to humans as a leaf vegetable; old leaves and stalks can be bitter but young leaves have a flavour similar to lettuce.