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Cox, Paul

Petrophytum caespitosum

Petrophytum caespitosum (Nutt.) Rydb.

Mat Rockspirea, Rocky Mountain Rockmat, Dwarf Spiraea, Petrophytum, Tufted Rockmat

Rosaceae (Rose Family)

Synonym(s):

USDA Symbol: PECA12

USDA Native Status: L48 (N)

Rocky Mountain Rockmat or Dwarf Spiraea�s tiny, closely packed, single-veined leaves of silver form dense cushions 1 in. high and up to 2-3 ft. across. The prostrate mat spreads from a woody rootstock to "pour" over rock surfaces. Tiny, white flowers are congested in short, bottle brush-like spikes. This plant resembles stone with thick, impenetrable, gray cushions above which white flowers bloom in dense racemes.

This strange plant is found almost exclusively in the inhospitable habitat of barren rock, clinging by stout roots jammed in crevices. There are only two other species. Chelan Rockmat (P. cinerascens), in central Washington, and Olympic Mountain Rockmat (P. hendersonii), in the Olympic Mountains of western Washington.

From the Image Gallery

2 photo(s) available in the Image Gallery

Plant Characteristics

Duration: Perennial
Habit: Herb
Fruit Type: Aggregate , Follicle
Size Notes: Ground-hugging mat with flower spikes up to about 8 inches tall.
Fruit: Aggregated follicles.

Bloom Information

Bloom Color: White
Bloom Time: May , Jun , Jul , Aug , Sep , Oct

Distribution

USA: AZ , CA , CO , ID , MT , NM , NV , OR , SD , TX , UT , WY
Native Distribution: N.e. OR to CA, e. to the Black Hills & TX
Native Habitat: Granite or limestone rock crevices from foothills to alpine summits

Growing Conditions

Light Requirement: Sun
Soil Description: Rocks or coarse, gritty soil.
Conditions Comments: This species takes well to the rock garden.

Propagation

Description: Can be propagated from small cuttings or seed.
Commercially Avail: yes

Find Seed or Plants

Find seed sources for this species at the Native Seed Network.

From the National Organizations Directory

According to the species list provided by Affiliate Organizations, this plant is on display at the following locations:

Santa Barbara Botanic Garden - Santa Barbara, CA
Native Seed Network - Corvallis, OR

Web Reference

Webref 38 - Flora of North America (2019) Missouri Botanical Garden, St. Louis, MO & Harvard University Herbaria, Cambridge, MA.
Webref 23 - Southwest Environmental Information Network (2009) SEINet - Arizona Chapter

Additional resources

USDA: Find Petrophytum caespitosum in USDA Plants
FNA: Find Petrophytum caespitosum in the Flora of North America (if available)
Google: Search Google for Petrophytum caespitosum

Metadata

Record Modified: 2023-02-22
Research By: TWC Staff

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