San Diego Thorn Mint [Acanthomintha ilicifolia (Gray) Gray]
Listing CNPS List 1B R-E-D Code 2-3-2
State/Federal.Status -- CE/C1 LAMIACEAE Apr.-Jun.
Global Rank G1 State Rank S1.1
Distribution: San Diego County; Baja California, Mexico
Habitat: Grassy openings in the chaparral or sage scrub with friable or broken clay soils are the preferred habitat of this species. These small clay lenses may be associated with Las Posas or San Miguel-Exchequer soils. Typically the microhabitat favored by San Diego Thorn Mint is quite distinctive. Only spring annuals, bulbous perennials, and a few herbaceous elements such as Sisyrinchium bellum are found with this tiny annual. The introduced Centaurea melitensis often grows with it, and these spiny and superficially similar seedlings can make a quick census for the mint difficult. All sites examined have a crumbly and/or deeply fissured soil which noticeably compresses, even during the dry season, when one treads on this friable terrain.
Known Sites: Known populations of San Diego Thorn Mint are typically small. Extant colonies are found in Mission Trails Park on the proposed Jackson Drive alignment, on the northern bluffs of Poggi Canyon east of the 1991 terminus of Orange Avenue (an eastward extension is proposed), west of the Las Brisas Mobile Home Park, north of Mustang Country Road in Alpine, and at several locations near the summit of McGinty Mountain. A large population with extensive habitat was found south of Poway Road and east of Interstate 15 near an abandoned wastewater treatment plant, but was heavily impacted by a haul road virtually bisecting the site in early 1989. A new site was reported 1 mile south of the community of Suncrest in a broad native grassland on a plateau overlooking the Sweetwater River. A population on a barren hillside north of the eastern arm of Lower Otay Reservoir was found in 1990; a second population was reported from a nearby hillside to the north. West of Rolling Ridge Road and just south of Proctor Valley Road near Chula Vista a population of several hundred plants was observed in 1994 with some thornmints growing to twenty-six inches in height. Also, recent reports are of San Diego Thorn Mint isolated within a new residential development in Rice Canyon, Chula Vista; east of Victoria Lane and north of Otto in Alpine; South of Pomerado Road in southeastern Poway; near Black Mountain Road; at the Carillo Ranch near El Fuerte Street in Carlsbad; near Lone Jack Road in Olivenhain; and a site near South Grade Road and Interstate 8. An old biological survey report notes a site near Black Mountain Road and south of Horseman's Park. Herbarium specimens at the San Diego Natural History Museum record occurrences at an old burn on Poway Grade, near vernal pools by Las Posas Road in San Marcos, near the Carlsbad Raceway in San Marcos, at Bonita, a slope at the mouth of Mahogany Canyon, the mouth of Alvarado Canyon, in the Merriam Mountains southeast of Twin Oaks Valley, north of San Diego State, on Viejas Grade at Poser Mountain, at the Encinitas Heritage Park, at La Mesa Springs, west of Interstate 15 near Black Mountain Road on the Alva Ranch, at the Junction of 6th Street near the junction of Highway 395, the junction of Encinitas road and Rancho Santa Fe Road, and 4 miles south of Vista. Very old collections from herbariums outside the County include a dry hillside near El Nido, a field near Sweetwater, University Heights, Mission Valley, 0.5 mile east of San Diego State, a mesa 2 miles west of San Diego State, in Spring Valley, Chollas Mesa, Paradise Valley in National City, a dry hillside near Bernardo, and between Evans Point and Letterbox Canyon near El Camino Real Road. Data Base records note sites north of Lusardi Creek and over a mile from the confluence of San Dieguito River, 0.25 mile northwest of the mouth of Mission Gorge, north of La Moree Road and south of Highway 78 east of San Marcos, 0.5 mile south of the San Dieguito River near Artesian Road, 0.7 mile east of Rancho Santa Fe Road and Avenida Esteban, 1 mile east of Rancho Santa Fe Road near Desert Rose Way, on Viejas Mountain, the southwestern corner of Linda Visa Street and Bent Street in San Marcos, east of Carlsbad 0.25 mile west of Letterbox Canyon and 0.7 mile east of Evans Point Peak, 0.6 mile southwest of Rancho de Los Quiotes near El Fuerte Street, at the mouth of a canyon south of Proctor Valley Road west of Upper Otay Reservoir, at the corner of La Mirada Drive and Pacific Street in San Marcos, north of Los Peñasquitos Creek near an overlook for the Peñasquitos Canyon Park's small waterfall, the southeast corner of the junction of La Costa Avenue and Rancho Santa Fe Road, near Rancho Bernardo 1.5 mile west of Interstate 15 and above Thornmint Court, on a ridgeline at Lux Canyon in Encinitas, northeast of Gooden Ranch in Sycamore and Slaughterhouse Canyons west of Highway 67 and northwest of Moreno Valley, near Mission Gorge by Colina Dorado Street and Tierra Santa Boulevard. A site east of Twin Oaks Valley north of San Marcos in the Merriam Mountains is apparently no longer extant. The species is now likely extirpated at many of these historical sites.
Collected south in Baja California to 30 39' North on a ridge 6 mile northeast of Las Escobas by Moran (SD 951509). San Diego Thorn Mint is lightly collected with 11 Baja specimens deposited in the San Diego Natural History Museum's herbarium.
Status: San Diego Thorn Mint is substantially declining; many historical populations are extirpated and many extant sites are endangered by urban pressures. This species may be difficult to detect in the fall and winter unless one is familiar with its distinctive microhabitat. These tiny plants remain rigid and retain their distinctive shape well into the dry season. While this annual can be raised from seed, suitable friable clay microhabitats are quite uncommon and place strict limitations on establishment of new populations. Given the very small total area utilized by a typical population, and the tenuous long-term prognosis for such limited microhabitats in a heavily urbanized region, all existing sites are recommended for protection.
Copyright © May 1994 Craig H. Reiser.
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