Blue Cardinal Flower (Great Lobelia; Blue Lobelia)
Perennial with unbranched stalks or branching toward the tip, often slightly winged. Flowers in leaf axils of upper stem leaves, having the typical lobelia shape with a 2-parted upper lip and 3-divided lower lip, to 1 inch long; color light or dark violet, light or dark blue or lavender, rarely white. Blooms August-October. Leaves alternate, light green, narrowly lance-shaped, 2–6 inches long, sometimes with a few irregularly spaced teeth.
Similar species: Missouri has 5 species of Lobelia, 4 of which are common: Cardinal flower (L. cardinalis) has bright red flowers. Spiked lobelia (L. spicata) has pale blue or white flowers; Indian tobacco (L. inflata) is similar, but its ovaries become inflated as the seeds ripen.