San Mateo Thornmint

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San Mateo Thornmint

Acanthomintha obovata duttonii

ListedSeptember 18, 1985
FamilyLamiaceae (Mint)
DescriptionSmall mint with slightly toothed oblong leaves and creamy white flowers.
HabitatGrassy slopes in soil derived from ser-pentine rock.
ThreatsResidential development, off-road vehicles.


The San Mateo thornmint, Acanthomintha obovata ssp. duttonii, is a strong-scented annual herb. It branches from near the base, and grows to a height of 4 to 7 in (10-18 cm). Its leaves are paired and oppositely arranged along four-sided stems. The fruit is a group of four small nutlets. The oblong leaves are 0.25 to 0.75 in (6 to 18 mm) long and slightly toothed. The upright flowers are creamy white with a rose to purplish wash on the notched lower lip. Each flower is surrounded by spiny leaf-like bracts at the base of the flower. Germination occurs from November to December with leafing in mid-March; budding in late March; anthesis in April to June; fruiting from May to June; and seed dispersal in June. The San Mateo thornmint is thought to be insect-pollinated, probably by species of bees. Individual plants of the San Mateo thornmint can produce large numbers of seeds (nutlets). In one study, the survival of plants until reproduction was more than 50%. The nutlets require six months of dormancy after production to germinate. Conditions such as local climate, soil, and herbivory may profoundly influence germination rate, seedling establishment, and survivorship in nature.

The San Mateo thornmint is closely related to Acanthomintha obovata, the San Benito thornmint, and to Acanthomintha ilicifolia, the San Diego thornmint. It differs from A. obovata and other species in the genus in lacking needlelike spines on the margins of the upper leaves, in having pink-red anthers (male reproductive flower parts), and in its generally unbranched habit with a solitary head-like flower cluster per stem.


The San Mateo thornmint is endemic to serpentine soils of chaparral and valley and foothill grassland. It occurs on grassy slopes and flats with deep, heavy-clay soil inclusions. The specific soil habitat in which San Mateo thornmint occurs is extremely limited. The deep, clay, serpentine-influenced soils have a low calcium/magnesium ratio, high percent moisture, and high cation exchange capacity. More typical rocky serpentine soil surrounds the areas. Soils derived from serpentine rock are also high in nickel, and cobalt. Temperature is moderate year-round. On average, about 140 days out of each year are overcast with dense fog.


The San Mateo thornmint is endemic to San Mateo County, California. The species was never collected outside a narrow strip approximately 6 mi (10 km) long, extending from Woodside north to Lower Crystal Springs Reservoir. Three historical populations have been extirpated.

The San Mateo thornmint survives in only two natural populations and one introduced population. The natural populations are separated by approximately 0.6 mi (1 km) in Edgewood County Park and an adjacent area called the Triangle. The only large population, in Edgewood County Park, is a remnant of a more extensive population that was damaged by motor-vehicle use. Edgewood County Park also contains a small subpopulation downslope from the main population. The most recent estimates of the total number of reproductive individuals in the Edgewood Park population range from 9,660 in 1991 to 53,136 in 1994. The population size dropped to 20,931 in 1995 and again in 1996.

Since its discovery in the late 1980s, the Triangle population has typically contained fewer than 100 plants, having fewer than 20 plants in 1987, 34 plants in 1994, and 23 plants in 1995. The plants observed in 1994 and 1995 occupied an area of no more than 2.2 sq ft (1 sq m). Most of the plants were small and unlikely to produce many flowers. The Triangle site may have always been composed of few individuals, but soil characteristics suggest that the Triangle may contain unoccupied, but potential habitat.


The range of San Mateo thornmint is limited by its rare and specific habitat. Most suitable habitat has been destroyed by urbanization. Urbanization extirpated two populations, and road construction may have destroyed a third.

The Edgewood Park population is on land owned by San Mateo County that has been designated a natural preserve. The population is approximately 50 yards (45 m) downslope from a residential development, and used to be more broadly distributed on the slope prior to the expansion of the subdivision. Hydrologic changes have probably been caused by upslope house and road construction. Vandalism and off-road vehicle damage have also occurred. Recreational disturbance still occurs in the area, and trail development may be a threat. The San Mateo thornmint at Edgewood County Park could also easily be eliminated if a fire were to occur in its vicinity a fire line and/or use of heavy equipment occurred on the slope occupied by the species.

The Triangle population is on land managed by the San Francisco Water Department. At one time, the Department had fenced the site and was protecting against the use of pesticides. Trail construction on San Francisco Water Department lands in the Triangle could threaten all the rare plants in the area, including the San Mateo thornmint. Because the rare plant is currently limited to only one substantial population that occupies less than 645 square feet in Edgewood County Park, it is highly susceptible to chance events. An event that substantially damaged the Edgewood County Park population could prove disastrous for survival of the species as a whole.

The status of San Diego thornmint and its habitat in northwestern Baja California, Mexico, is not well documented. The species is known to occur as far south as Las Escobas near San Quintin, Mexico, but its distribution in Mexico is spotty. The San Diego Natural History Museum has herbarium specimens of A. ilicifolia from nine localities in Baja California, Mexico; however, little information is available on the numbers of individuals or specific threats. One population near Tecate, Mexico is threatened by an adjacent clay mining operation. This northern region represents one of the most severely impacted areas in Baja California, and many of the same factors (urban and agricultural development) that have affected the U.S. population are threats in the Mexican range.

Conservation and Recovery

Recovery of San Mateo thornmint must first focus on protecting and managing the two remaining populations by working with San Mateo County and the San Francisco Water Department to ensure the long-term survival of the species on their lands. Another high priority in recovery efforts for San Mateo thornmint is collection and banking of seed in the Center for Plant Conservation. Although some seed has already been stored, further collections are prudent to guard against extinction of the species from catastrophic events and to provide material for enhancement efforts in existing populations, repatriations, or introductions to new sites.

The larger of the two remaining populations occurs in Edgewood Park, which San Mateo County intends to manage as a natural preserve. San Mateo County personnel are aware of the special status plant species at Edgewood, but the San Francisco Water Department has no specific management goals for rare plants at this time.

Of the 32 extant populations of the San Diego thornmint, 11 are considered major populations supporting over 3,000 individuals each. Four of these major populations are located within the Multiple Species Conservation Program planning sub-region of southern San Diego County, California. Two of these, at Sabre Springs (private ownership) and Sycamore/Slaughterhouse canyons (San Diego County ownership) are adequately conserved by the Multiple Species Conservation Program. Another population, Asphalt Inc. (private ownership), is in the Multiple Species Conservation Program outside the Multiple Habitat Preserve Area, but will receive significant conservation benefits within the Metro-Lakeside-Jamul segment of the Multiple Species Conservation Program of the County of San Diego. The last of these four populations, Otay Lakes Northeast (private ownership), is not adequately protected. The remaining seven major populations are located either north or east of the Multiple Species Conservation Program subregion. Of these seven major populations, four are located within lands managed by the Forest Service (on Viejas and Poser mountains). The three remaining major populations and the majority of the smaller populations are on lands managed by private landowners.

One population of San Diego thornmint is on land managed by The Nature Conservancy and four populations occur in the Cleveland National Forest. These populations, however, are vulnerable to habitat degradation resulting from illegal dumping, trampling, erosion, and off-road vehicle activity. Roads adjacent to populations in the vicinity of McGinty Mountain and Penasquitos Canyon provide easy access for foot traffic and off-road vehicle use.

The San Diego thornmint has benefited from State listing. Since the species was listed in 1982, direct impacts to the species from development projects have been reduced. The configuration of remaining populations, however, is not conducive to long-term conservation; in many cases the development footprint is immediately adjacent or in proximity to A. ilicifolia populations. Consequently, habitat is degraded and risks from non-native plant replacement, trampling, fragmentation, and isolation increase.

In 1991, the State of California established the Natural Communities Conservation Planning program to address conservation needs of natural ecosystems throughout the State. The focus of the current planning program is the coastal sage scrub community in southern California, although other vegetation communities are being addressed in an ecosystem approach. The San Diego thornmint is currently covered under the Multiple Species Conservation Program and the Central/Coastal Subregional Natural Communities Conservation Planning/Habitat Conservation Plan (Central/Coastal Natural Communities Conservation Planning) of Orange County, California, and is being considered for inclusion as covered species under the Multiple Habitat Conservation Plan.

In 1991, seeds collected from Edgewood Park were sown at Pulgas Ridge in two subpopulations, one north-facing and one south-facing. The seeds were taken from plants that represented the range of sizes and microenvironments of plants in the natural population.


U.S. Fish and Wildlife Service
Fish and Wildlife Office
260 Southeast 98th Avenue, Suite 100
Portland, Oregon 97266-1398
Telephone: (503) 231-6179
Fax: (503) 231-6195


Abrams, L. 1951. Illustrated Flora of the Pacific States. Stanford University Press, Palo Alto.

Niehaus, T. 1977. "Rare Plant Status Report for Acanthomintha. " U.S. Fish and Wildlife Service, Portland.

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

U.S. Fish and Wildlife Service. "San Mateo Thornmint Recovery Plan." San Diego, Ca.