Common Name: mat rockspirea

Duration: Perennial

Nativity: Native

Lifeform: Subshrub

General: Low, mat-forming shrub or subshrub, often spread over rock faces; stems 2-25 cm tall, much-branched; herbage silky pubescent.

Leaves: Numerous crowded cauline leaves, alternate and sessile, linear-spatulate to oblanceolate, 3-30 mm long, 2-4 mm wide, margins entire, apex obtuse, densely covered with appressed, silky hairs.

Flowers: White, drying to a brownish color, in dense cylindric spike-like racemes, borne on erect, leafy-bracted scapes up to 10 cm tall; flowers numerous, small; pedicels 0.5-1.5 mm long; hypanthium top- to cup-shaped, somewhat silky pubescent; sepals ovate to triangular, 1-1.6 mm long; petals oblanceolate, 1.6-2 mm long, white, apex rounded, truncate, or shallowly notched, often irregularly toothed; stamens about 20; style pilose.

Fruits: Follicle, 1.2-1.6 mm long, pilose.

Ecology: Found on rock outcrops, crevices, canyon walls, and cliffs, from 1,500-8,000 ft (457-2438 m); flowers June-September.

Distribution: Western and Southwestern US.

Notes: Distinct as a low-growing shrub with thick wody stems which roots in rock cracks and spreads out along cliff-faces. It is often seen on limestone substrate, but is not obligate to that rock type. The genus name has often appeared in the literature with the ending -um. Host plant for the spring azure butterfly.

Ethnobotany: The Navajo use this plant ceremonially.

Etymology: The genus name translates to "rock plant," with petro- meaning rock, and -phyton meaning plant; caespitosum refers to the dense growth of stems and leaves.

Synonyms: Petrophytum caespitosum

Editor: FSCoburn 2015, AHazelton 2015