Achillea x lewisii

 A low carpet-forming species of yarrow, quite different from the taller types grown as border perennials. Suitable for edging a sunny border, but don't let t get swamped. Best in the rock garden or alpine trough garden. It forms a low carpet of cute olive-green leaves, producing small clusters of primrose-yellow flowers on taller stems in early summer. Sometimes used as a groundcover in small areas. Removing faded flowers will encourage more buds to form.  Dislikes heavy clay soil. Ideally divide every 2 to 3 years, in spring or early autumn, but mine has been going for years, slowly in my tough rock garden, uncared for and untrimmed.