Leonard and I were first introduced to Petrophytum caespitosum in October while visiting Great Basin National Monument in Nevada as “rock spiraea”. It was growing along the Mountain View Nature Trail.  Hmmm. . . We were familiar with a “rock spiraea” growing nearer to our house in NE California but its scientific name  is Holodiscus discolor var glabrescens. Therein lies the confusion that can be created by the use of colloquial or common names.

Other common names for P caespitosum include mat rock spiraea and rockmat. I decided to call this native perennial rockmat so as, in my own mind, not to confuse it with the “rock spiraea” with which I am more familiar.

Rockmat, a member of the Rose Family (Rosaceae), is found in Texas, South Dakota and the Rocky Mountain States westward, except for Washington. Its habitat is amid limestone, dolomite or limey sandstone rocks in mountainous areas where it attaches to or in rock crevices with stout roots.

Rockmat is a low, mat-forming shrub that creeps over the rocks. The stems are thick and short, grow horizontally and are densely covered in rosettes of greyish-green, lance-shaped leaves. The leaves have no marginal teeth.

The inflorescences are spike-like clusters of flowers at the ends of erect peduncles (stem supporting an inflorescence). The tightly clustered rockmat flowers are white to light pink. They have five tiny petals, five hairy pistils and many long, whiskery stamens. The dry peduncles persist for many months after flowering and are often more obvious than the flowers.

Rockmat fruits are follicles containing two to four smooth, brown, linear seeds.

The genus name, Petrophytum, from the Greek “petro/rock” and “phyton/plant” refers to rockmat’s rocky habitat. The species name means “mat like form”. Synonyms for P caespitosum are Spiraea caespitosum and Eriogynia caespitosum.

Since I never did a post on H discolor var glabrescens, the “rock spiraea” more familiar to me, that will be the topic of my next post.


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1 Response to Rockmat

  1. tonytomeo says:

    I NEVER would have guessed that to be in Rosaceae! There is nothing that looks right about it. Even the close up of the bloom (with the penny) is unfamiliar.

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