Mat Rock Spiraea
Petrophytum caespitosum
Mat Rock Spiraea (Petrophytum caespitosum) is a low native shrub in the rose family that grows as a tight, silver-gray mat on bare rock, with dense spikes of tiny white flowers on short stalks in summer. Its range reaches east to the Black Hills. Photographed in the Black Hills.
Free photos from other sources
| Family | Rosaceae · Rose |
|---|---|
| Status | Native shrub |
| Blooms | June into August |
| Height | Mat about 2.5 cm (1 in) high; flower stalks to about 20 cm (8 in) |
| Where | Cracks and bare faces of limestone and other rock, foothills to alpine summits |
| Also called | Rock Spiraea, Mat Rockspirea, Rocky Mountain Rockmat, Tufted Rockmat, Dwarf Spiraea, Rockmat |
Roots, seeds & germination
Root: A stout taproot anchors it in cracks in the rock.
Seed: Dry, many-seeded follicles (small pods) about 2 mm long.
How do you identify Mat Rock Spiraea?
Look for a flat, silver-gray mat stuck tight to the rock, made of narrow, spoon-shaped, silky leaves under 2 cm long, with dense spikes of tiny white flowers and long stamens. Dried flower spikes stay on for many months and are often easier to see than the flowers. It looks like a Spiraea, a related genus of woody shrubs, but grows as a flat mat and has different leaves.
When does it bloom?
June into August.
Where does it grow?
Rock crevices and cliff faces in full sun, mostly limestone, dolomite and limey sandstone, and less often granite. Also rocky outcrops and talus slopes (loose rock below cliffs).
Other flowers to compare
Sources: Lady Bird Johnson Wildflower Center: Petrophytum caespitosum; USDA Forest Service Plant of the Week: Rock Spiraea; Flora of North America: Petrophytum caespitosum; Bean's Trees and Shrubs Hardy in the British Isles: Petrophytum caespitosum.