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Golden Gate National Recreation Area

San Mateo Thornmint

San Mateo Thornmint
Acanthomintha obovata ssp. duttonii

Photo © Toni Corelli

Home >> Species >> San Mateo Thornmint

San Mateo Thornmint
Acanthomintha obovata ssp. duttonii

The San Mateo Thornmint is an aromatic annual herb in the mint (Larniaceae) family.  It is a small annual species, usually less than 20 cm tall.  It blooms from April through June.  The species only grows on moist seeps in serpentine grasslands in San Mateo County.  The flowers are white and tinged with lavender and grow in tight clusters surrounded by a nearly round, small, leaf-like or scale-like structures (bracts) with prominent spines. No other species with a similar appearance grows within the range of San Mateo Thornmint.

The species was protected as endangered under the federal Endangered Species Act in 1994.
The San Mateo Thornmint is “endemic” to San Mateo County, which means it cannot be found naturally anywhere else in the world.   It also means that the species has a limited geographic range.  Thus the loss of even a few populations of this species could spell trouble for its continued existence.

Unfortunately, that is precisely what happened.  At least five historical occurrences of the species have already been destroyed by development or habitat loss.  Only one natural population exists today, and this population is within the legislative boundary of the GGNRA. 

This single remaining population has been in serious decline over the last few years. An estimated 53,000 individuals in 1994 had dropped to only 500 in 2007. The managers of the area where this population is found have been unable to arrest or adequately explain why this decline has occurred. The population is now so imperiled that the managers believe that even a low number of additional visitors could damage the plant by compacting soil or introducing weed seeds.

Because the site is off-trail in a remote area, sightings of San Mateo thornmint will not be counted in the Big Year contest as viewing this species would violate the Endangered Species Big Year Ethical Principles. No trips will be scheduled to see the species this year.

Conservation Action Item

The San Mateo Thronmint is threatened by urbanization, alteration of drainage patterns, and presently by non-native plant invasion.  Italian ryegrass, Lolium multiflorum, has been particularly problematic at the San Mateo Thornmint’s last remaining site. You can help the San Mateo Thornmint recover by volunteering at Edgewood County Park. To be put on a list and find out how, click here.

Trips

Saturday, May 17, 2008, 9:00 a.m. - Noon: Join Friends of Edgewood Park and the Edgewood Weed Warrior program in restoring habitat for the San Mateo thornmint. Wear sturdy shoes, long pants and bring water. RSVP required: email Drew Shell at dshell@microsoft.com. Meet at the West Kiosk. Click here for directions.

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Previous Comments

Posted by: Cheryl Erb | 2008-05-23 11:23:18

It's unbelievable that San Mateo is Thornmint’s last remaining site. What a shock. It makes me realize that the climate changes are severe in a lot of places and how much they can influence some plants. This is another great reason why non-native species and invasive weeds need to be taken seriously. Everyone can help, even if it's in their own little garden space. Thank you, Cheryl