Soft pink four-petalled flowers scattered along slender arching stems so fine that the plant seems to be all flower and no structure. The common name is whirling butterflies and it earns it; the smallest breeze sets the whole thing trembling.
Its real job is softening. Put it in front of something blocky, echinacea, echinops or dahlias, and the airy stems throw a veil across the rigid shapes and make the whole planting look less arranged. Very few perennials do that, and fewer do it from June right through to October.
It comes from the prairies of Texas and Louisiana, and its requirements follow from that. It is thoroughly drought-tolerant once established and will grow happily in a pot that gets forgotten. What kills it is a wet winter in heavy ground. In a well-drained sunny spot it is reliably hardy here; in cold clay it rots at the crown and does not come back, so if that is your soil, grow it in a container. It also breaks dormancy very late in spring, so do not write it off in April.
