Petrophytum caespitosum aka Rock Mat

Petrophytum caespitosum is a low, evergreen shrub in the rose family, Rosaceae. Its common name is mat rock spiraea. Its native range spans the western and central United States. It extends south into northeastern Mexico. Kew records it as a plant of temperate regions. It is not native to Britain or Ireland. Its low form differs from that of an upright shrub.

The plant forms a dense mat of stems and leaves. Virginia Tech gives its height as 0.5–0.67 feet [0.15–0.20 metres]. This figure excludes the flower spikes, which rise above the foliage. Mats can spread several feet [roughly a metre or more] across. These figures describe its form, rather than a stated garden size range. The branches lie close to the ground instead of forming a tall crown. Gray bark breaks into uneven scales. A thick cover of leaves clothes the gray twigs.

The leaves are simple, with no separate leaflets. Each has a narrow base and a broader, rounded tip. Botanists call this shape spatulate. They measure about 0.02–0.04 feet [0.006–0.013 metres] long. Their edges lack teeth, though they may show a pink tint. The leaf surface looks gray-green and feels silky. Leaves grow in an alternate pattern along the stems. Their close spacing can make them look whorled. The evergreen foliage keeps the mat leafy beyond the flowering season.

Small flowers appear in late summer and early fall. Their colour ranges from white to pale pink. They crowd into narrow, spike-like clusters on single upright stems. Each cluster measures about 0.08–0.25 feet [0.025–0.076 metres] long. The flower-bearing stems measure about 0.17–0.25 feet [0.05–0.08 metres] tall. The crowded blooms give each cluster a bottle-brush shape. These upright spikes contrast with the low mat beneath them.

Small, light-brown follicles follow the flowers in tight clusters. The fruit clusters retain a narrow, spike-like form several inches long. They ripen in late fall and remain through winter. Thus, the plant has visible fruit after its flowers fade. The supplied sources do not set out garden care needs or hardiness limits. They also give no evidence about pests, diseases, edible uses, or toxicity.