This biennial wildflower is 4-12' tall, developing one or more leafy stems from a taproot. These stems are erect, ascending, or sprawling; they are light green, glabrous, angular, and usually unbranched (although they may branch after the blooming period). Numerous alternate leaves occur along each stem that are ascending to widely spreading. Individual leaves are narrowly oblong or narrowly oblong-oblanceolate and smooth along their margins; they are medium green, glabrous, and sessile. Each leaf has a single prominent vein. Each stem terminates in a spike-like raceme of purple flowers about ¾-4' in length. The central stalk of the raceme is light green to purplish green and glabrous. Each flower is about ¼' long and across when it is fully open, consisting of 3 petals, 5 sepals, several inserted stamens, and a pistil with a single style. Two sepals are enlarged, rosy pink to purple, and petaloid, forming a pair of lateral wings. The remaining three sepals are smaller in size, light green to purple, and ovate in shape. The 3 petals form a fringed tubular structure that surrounds the stamens and style; they are rosy pink to purple, often becoming more white toward their tips. The short glabrous pedicels of the flowers are light green to purple, slender, and often nodding. Sometimes a few cleistogamous (self-fertile) flowers develop toward the base of the raceme; they are bud-like and inconspicuous. The blooming period occurs from early to mid-summer and lasts about 3 weeks. Only a few flowers are in bloom at the same time. Afterwards, they are replaced by 2-celled seed capsules; each cell of a capsule contains a single hairy seed. In addition to the above-ground flowers, Purple Milkwort also produces cleistogamous flowers along underground stems. The root system consists of a taproot.