Purple owl's clover (Castilleja exserta) is an annual in the same species as the Indian paintbrushes. It was formerly included in the genus Orthocarpus, and some sources may still refer to it by that name. Although this species is variable in appearance and easily hybridizes with other Castilleja, it generally bears shaggy pink-purple or lavender flower clusters that resemble clover (though they are not related). The thin, erect leaves are usually tipped with the same color, giving the flower cluster the appearance of a paintbrush. It produces pods containing seeds.

The plant's showy appearance when in flower makes it a great fit for informal meadows and related landscapes. Like other related plants in the family, purple owl's clover derives some of its nutrients directly from the roots of other plants. Therefore, it is almost always planted, or grown from seed, with a perennial species from its natural plant community to serve as the host plant, such as a native grass. It takes full sun, prefers fast-draining soil but is tolerant of sand and clay. It does not need summer water.

The flowers attract bees and butterflies. It is a crucial host plant for the Bay Checkerspot butterfly, a threatened species in California, and hosts a likely total of 12 species of butterfly and moth.