Mat rockspirea, Rocky Mountain rockmat
From Idaho and Montana, south to California and west Texas
Up to 8 inches (the flower stalks)
Talus slopes, cliff ledges, rocky places; 3,600 to 10,000 feet
Spatulate, up to 0.7 inches long and 0.15 inches wide
June to September
Petrophyton caespitosum is by far the most widespread of four US species in this genus, the range extending from west Texas to Montana and California, yet it is still somewhat uncommon. The inflorescence may be narrow and elongated or wide and branched, and contains up to 100 closely-spaced flowers, each subtended by a thin green bract. Flowers have five green, lance-shaped, outwards-pointing sepals and five slightly larger white petals, plus five pistils and around 20 white stamens, which are up to twice as long as the petals. The anthers are colored pale pink or light brown.
Stems are short, just a couple of inches, and generally grow sideways, bearing closely spaced leaves; plants usually form dense, low mats, several feet wide. The lightly hairy, green/green leaves form tight rosettes. The leafless flower stalks rise up to 8 inches. Plants grow in rocky places, sometimes anchored in crevices in cliffs with the stems hanging downwards.