This introduced perennial plant is 2-5' tall, branching frequently below the inflorescence. The stems are variably hairy, becoming woody and glabrous below. The leaves are usually opposite, less often whorled in 3's; some of the upper leaves in the inflorescence may be alternate. These leaves are up to 4" long and ¾" across, becoming smaller as they ascend the stems. They are lanceolate, smooth along the margins, slightly hairy (especially the upper leaves), and clasp the stems. The upper stems terminate in long spikes of flowers about ½2' long. Each flower is about ½1" across, consisting of 6 purple petals, a green tubular calyx, 6 or more stamens, and a pistil with a stigma that is green and knobby. Each wrinkly petal has a dark purple line toward its base. The hairy calyx has 5 teeth at its apex and several veins along its length. Sometimes the flowers have fewer than 6 petals, and the relative length of their pistils and stamens is variable (in this regard, there are 3 different forms of flowers). The flowers are sessile against the flowering stalks, or they have very short pedicels. The blooming period occurs from mid-summer to early fall, and lasts about 2 months. Each flower is replaced by a small seed capsule that is surrounded by the tubular calyx. This capsule contains many tiny seeds that can float on water or be blown about by the wind. The root system is shallow and fibrous, frequently forming offsets by rhizomes. This plant often forms colonies, and can spread by its seeds, rhizomes, or segments of the roots and stems.