Tall, robust annual or biennial with a straight stalk, unbranched or sparsely branched near the top, often persisting in winter. Flowers tubular with 5 large, spreading lobes, saucer-shaped (not bell-shaped), arising singly or in clusters of 3 on a very long spike, light blue to rarely white. Flowers have a white ring at center. The style is curved, purple, and projects well beyond the flower. The flowers often open without any apparent sequence along the stalk. Blooms July-October. Leaves alternate, 3–6 inches long, lance-shaped, sessile or with very short stems. Sap is milky.
Similar species: Missouri has 4 species in the genus Campanula. Two of them are fairly uncommon natives. The fourth, creeping bellflower (C. rapunculoides), is an introduced Eurasian plant that can spread aggressively via creeping rhizomes. It has a bell-shaped corolla, and the style is straight and only slightly exserted from the corolla. It is scattered mostly in the northwestern quarter of Missouri.