San Mateo Thornmint
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Sacramento Fish & Wildlife Office

Species Account

SAN MATEO THORNMINT
(Acanthomintha obovata ssp. duttonii)

CLASSIFICATION: Federal Endangered Species (Federal Register 50:37863 (pdf); September 18, 1985)

CRITICAL HABITAT: None designated.

RECOVERY PLAN:  Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area, September 30, 1998 (pdf format)

DESCRIPTION:

San Mateo thornmint (Acanthomintha obovata ssp. duttonii) is an aromatic annual herb of the mint family (Lamiaceae). The 1½ to 8-inch high plants are typically unbranched, though most populations contain some plants branched from near the base. The plants have square stems and opposite leaves. The oblong to egg-shaped leaves are 0.3 to 0.5 inch long and may have toothed margins.

Flowers, which bloom from April through June or July, are white or sometimes tinged with lavender and grow in tight clusters surrounded by almost round, prominently spined, small leaf-like or scale-like structures (bracts). No other species with a similar appearance grows within the range of San Mateo thornmint. See Hickman (1993) in General Information about California Plants, below, for a detailed description of the species under the name Acanthomintha duttonii.

DISTRIBUTION:

San Mateo thornmint is restricted to serpentine soils of chaparral and valley and foothill grasslands in San Mateo County. The species occupies slopes and flats with deep, heavy-clay soil inclusions. The only remaining large population, in Edgewood County Park, is a remnant of a more extensive population damaged by motor-vehicle use. Edgewood County Park also contains a small subpopulation. There is an introduced population at Pulgas Ridge.

Serpentine soils are formed from weathered volcanic (ultramafic) rocks such as serpentinite. dunite, and peridotite. These soils provide a harsh environment for plant growth. Several factors contribute to the inhospitability of serpentine soils to plant growth including: 1) a low calcium-magnesium ratio; 2) lack of essential nutrients such as nitrogen, potassium, and phosphorous; and 3) high concentrations of heavy metals (mineral toxicity). However, species such as San Mateo thornmint have adapted to serpentine soils and require them to survive.

SPECIAL CONSIDERATIONS:

San Mateo thornmint is seriously threatened by urbanization, which extirpated two populations. Road construction may have destroyed a third. The extant populations are threatened by development, off-road vehicles and vandalism.

This species was listed as endangered by the California Department of Fish and Game in July 1979, under the name Acanthomintha duttonii. The California Native Plant Society has placed it on List 1B (rare or endangered throughout its range), also under this alternate name.

REFERENCES FOR ADDITIONAL INFORMATION:

Jokerst, J.D. 1991. A revision of Acanthomintha obovata (Lamiaceae) and a key to the taxa of Acanthomintha. Madroño 38: 278-286.

Steeck, D.M. 1995. Reproductive biology of a rare California annual, Acanthomintha duttonii, and its congener, Acanthomintha obovata ssp. cordata. Master's thesis, University of California, Davis, CA.

Thomas, J.H. 1961. Flora of the Santa Cruz Mountains of California. Stanford University Press, Stanford, California.

U.S. Fish & Wildlife Service. 1985. Endangered and Threatened Wildlife and plants; Determination of Endangered Status for Acanthomintha obovata ssp. duttonii (San Mateo Thornmint). Portland, Oregon.

U.S. Fish and Wildlife Service. 1998.  Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area. Portland, Oregon.

General Information about California Plants

Photo credit: San Mateo Thornmint by Niall McCarten © 1986 California Native Plant Society Calphoto ID: 0000 0000 0502 0864

Prepared by Endangered Species Div., Sacramento Fish & Wildlife Office, U.S. Fish & Wildlife Service

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