Letterman's ironweed is the smallest of the garden-worthy ironweeds, a native of rocky outcrops, river scours, and floodplain edges in Arkansas and Oklahoma. Named for the American botanist George W. Letterman, it grows just 2 feet tall and equally wide, a tidy scale that makes it practical in gardens where its towering relatives would quickly take over. The upright habit stays honest in lean, well-drained soil; rich fertile ground encourages the stems to bend and arch in a way that suits no one. Plant it in full sun and let it find its footing.

The frilly purple blooms open in mid to late summer, precisely when the garden needs a bridge between the exuberance of early summer and the slower rhythms of fall. Butterflies, hummingbirds, and bees all visit reliably, and the contrast between its purple flowerheads and the yellow plumes of nearby goldenrods is one of the more satisfying color combinations late summer has to offer. Established plants tolerate occasional flooding and drought, a useful resilience given the variable weather of its native range in zones 4 through 9. Division or seed will multiply it readily, and the hollow stems are worth leaving through winter for the same nesting-bee reasons that apply across the genus.