The plant Leonard and I commonly call rock spiraea is a variety of Holodiscus discolor (oceanspray or cream bush). As I mentioned in my last post (“Rockmat” on 11-06-19), Petrophytum caespitosum is often also colloquially referred to as rock spiraea. The shrub I learned as and continue to call rock spiraea is Holodiscus discolor var glabrescens. There are ten or more varieties of H discolor and they intergrade so the taxonomy is complex. To further complicate the matter, a synonym for “this” rock spiraea is Holodiscus microphyllus var glabrescens and another common name is desert oceanspray.
A native, rock spiraea grows in dry, rocky areas and outcrops from 4,000 to 9,500 feet. It can be found throughout the West, growing to a height of about 3 1/2 feet.
Rock spiraea leaves are extremely variable in shape. They have short, if any, petioles and no stipules. The alternate, deciduous leaf blades exhibit a wedge-shaped base and the margins above the middle are toothed. Occurring on long, lateral twigs on older branches, rock spiraea leaves are dotted with glands and are lightly pubescent (hairy) with longer hairs on the margins.
Inflorescences are terminal and conical, often with leaves intermixed. The white or creamy flowers have five sepals and five petals. Like other members of the Rose Family (Roseaceae), rock spiraea has many (usually 15 to 20) stamens. The superior ovaries have two ovules and dense, bristle-like hairs. These hairs and the style remain persistent in the fruit. All the flower parts sit on a saucer-shaped hypanthium (cup that supports or holds the flower parts).
The infructescences, arrangement of fruits on the stem, persist on the plant late into and through the winter. Rock spiraea fruits are small brown achenes containing 5 seeds per flower.
Rock spiraea holds little food value for wildlife. Browsers consume rock spiraea only when other food is scarce. However, it does provide good cover for small mammals and birds. Insects pollinate rock spiraea flowers. The seeds are usually scattered by the wind, although small animals play a secondary role in seed dispersal.
The genus name, Holodiscus, derives from the Greek and means “entire disk”, referring to the unlobed hypanthium. The leaf blades are slightly lighter on the underside, which may be the origin of the species designation, discolor, from the Greek “dis”/two”. There is a second meaning to “dis” as a Greek prefix indicating negation or something without. Since the flowers are white, without color, that might also be the species derivation. “Becoming glabrous (smooth)”, the variety meaning, refers to the fact that although the leaves have hairs, the hairs are not numerous.
These specimens were photographed in June and July along CA Highway 299 E near the Cedar Pass Summit (Modoc County) or near the Mammoth Crater in Lava Beds National Monument (Siskiyou County CA).
Now that I think about it, I never did a post on oceanspray. I need to dig out my oceanspray pictures and correct that omission.