This perennial wildflower is ¾-2½' tall and either unbranched or sparingly branched. The central stem is light green or yellowish green, 4-angled, finely grooved along its sides, and hairy. Pairs of opposite leaves occur along the central stem; they are 1½-3' long and ½-1½' across, becoming more slender where the flowers occur. Leaf shape is lanceolate-oblong to ovate. Leaf margins are coarsely dentate or pinnatifid with cleft lobes that are tooth-like in shape. Upper leaves that are adjacent to the flowers tend to have fewer teeth or lobes than middle or lower leaves. The upper blade surface is yellowish green to dark green, hairless to sparsely short-pubescent, and glandular. The lower blade surface is more pale and hairy; hairs are concentrated particularly along the major veins. The leaves usually have short stout petioles (less than ¼' long), although upper leaves may be sessile. Dense sessile whorls of small flowers occur in the axils of the upper leaves. Individual flowers are 1/8' across, consisting of a short-tubular calyx with 4-5 teeth, a white corolla with 4 lobes, 2 fertile stamens, and an ovary with a style that is bifurcated at its tip. The calyx is light green or yellowish green and glabrous to sparsely pubescent; its teeth are linear-lanceolate (more than 2 times longer than across), tapering gradually to awn-like tips. Generally, the teeth of the calyx are longer than its tube. The lowest corolla lobe of each flower is purple-spotted. Linear-lanceolate bractlets are present at the bases of the flowers. The blooming period occurs from mid-summer to autumn, lasting 2-3 months. Only a few flowers are in bloom at the same time toward the apex of the plant. Each flower is replaced by 4 nutlets that together form a slightly concave surface within the persistent calyx; these nutlets are shorter than the calyx. Individual nutlets are about 1.5 mm. long, obovoid, and angular along their sides. The root system is highly rhizomatous, forming colonies of plants from vegetative offsets.