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San Mateo Thornmint


Acanthomintha obovate ssp. duttonii

By Cristal Niederer

Did you know there is a plant found only at Edgewood?

San Mateo thornmint is an annual herb from the mint family (Lamiaceae), found only in serpentine soils with heavy clay inclusions. This uncommon soil type is known as serpentine vertisol. It is listed as federally endangered, state endangered, and a CNPS List 1B.1. Since publication of the 1998 U.S. Fish and Wildlife Service recovery plan for serpentine soil species of the San Francisco Bay Area, two populations of this species have been extirpated.

There is currently only one known extant population, in Edgewood County Park and Natural Preserve. The specific location is kept secret to prevent accidental trampling of the small plants at this lone site.

This one population has been in decline for years. Pavlik and Espeland (1998) estimated over 53,000 individuals in 1994 and 5,289 in 1997. In 2008, only 249 individuals were found, confined to a few tens of square meters. This site has been densely invaded by Italian ryegrass (Lolium multiflorum), possibly exacerbated by nitrogen deposition syndrome. Most plants were found in the few relatively open areas within the historic habitat.

Species Description

Like many other mints, our thornmint has opposite leaves, a whorled or headlike infloresence at the top of the stem, and a bilabiate corolla. As with other Acanthomintha species, it has inflorescence bracts with sharp spines. It is distinguished from other congenerics by its generally unbranched stem, its fertile upper stamens, pink-red hairy anthers, and upper leaves with margins that are not spiny.

It grows up to 10 cm tall. Its simple leaves grow up to 1.5 cm long, with toothed margins. Flowers are sessile, with a fused, green calyx to 5 mm long. The corolla can reach 1 cm long, and is white with pink lobes. The roundish inflorescence bracts are up to 1 cm long, with sharp spines to 5 mm. Each flower can produce up to four small nutlets (Hickman 1993, Corelli 2004, Pavlik and Espeland 1998).

Restoration Project Summary

A restoration project began in 2008 with the goal of conducting habitat enhancement experiments at the existing site, conducting habitat suitability surveys for potential introductions, collecting and banking seed from the existing population, and initiating a seed increase program. As a direct result of this project, San Mateo thornmint numbers increased to 3,135 in 2010. Key actions completed to date include:

  • Experimental seeding and germination counts. 9,000 seeds were sown into a total of 30 scraped plots on December 2, 2010 at Edgewood. On January 27, 2011, a total of 2,225 seedlings were counted, representing a 24.7% germination rate and 74.2 plants per plot. On November 6, 2009, 12,500 seeds were sown into a total of 25 1-meter scraped, hand weeded, and control plots at Edgewood. On January 20, 2010, 3,777 San Mateo thornmint seedlings were counted, which represents a 30% germination rate. A third seeding treatment of 30 scraped plots using a total of 7,500 seeds took place December 9. 2014.
  • Population monitoring and seed collection. In 2009, before seeding took place, 395 individuals were censused, and 171 seeds were collected.In 2010, 3,135 individuals were censused, including 2,885 from the seeded plots. About 4,000 seeds were collected from the restored population at Edgewood. After a second seeding effort, 3,450 plants were counted in 2011. The population dropped again over the next three years (934, 875, then 608 individuals) as extremely dry weather and no seeding efforts took their toll. We are hopeful that additional seeding and more normal rainfall will help increase the numbers again.
  • Habitat enhancement experiments. Baseline vegetation data were collected at the existing site in April 2008, and the following enhancement treatments were completed: hand weeding, graminicide (herbicide that only kills grass), spring mowing, fall raking, and late fall scraping. Plots were reread in April 2009, and scraping was determined to be the most effective treatment for increasing bare ground and reducing thatch. No San Mateo thornmint have been found in the experimental plots, indicating a lack of seedbank response. Bare cover throughout the habitat has increased from 4.6% in 2010 to 31.0% in 2014. Nonnative cover has dropped from 45.5% to 18.0%.
  • Habitat suitability surveys and soil moisture studies. Several potential introduction sites on San Francisco Water District land were identified, and initial testing for soil composition and moisture show they are similar to the extant site.
  • Germination trials. In 2008, the UC Berkeley Botanical Garden produced 5,715 seeds from an initial investment of 240 seeds.
  • Seed increase. The UC Berkeley Botanical Garden has produced ~100,000 seeds at their nursery by planting seeds left over from an old experiment. These seeds will be used to supplement the existing population, and hopefully to introduce new sites nearby.
  • Seed banking. The seeds collected from the wild collection, plus some seeds from the seed increase project, will be sent to the National Seed Storage Laboratory in Fort Collins, CO, to supplement their existing collection of 1998 stock.
Staff and volunteers prepare seedling plots.

Staff and volunteers prepare seeding plots.
© 2014 Lech Naumovich Photography

Discussions are progressing between the U.S. Fish and Wildlife Service and the San Francisco Public Utilities Commission to initiate introductions on their property. This endangered plant is now moving toward recovery!

Latest News

On December 9, 2014, Creekside Science, San Mateo County Parks staff, and volunteers from Friends of Edgewood and the Santa Clara Valley California Native Plant Socienty (CNPS) Weed Warriors placed 7,500 seeds into prepared plots at the thornmint site at Edgewood Park.  See photos to learn more.


Corelli, Toni. 2004. Flowering Plants of Edgewood Natural Preserve, Second Edition. Half Moon Bay, CA: Monocot Press. Hickman, J. C., Ed. 1993. The Jepson Manual. Berkeley, CA: University of California Press.

Niederer, C. and S. B. Weiss. 2010. San Mateo Thornmint (Acanthomintha duttonii) Restoration Project at Edgewood County Park and Natural Preserve, Year 2, Quarter 4 Status Report and Draft Adaptive Management Plan, April 30, 2010. Report submitted to the U.S. Fish and Wildlife Service. 95 pp.

Pavlik, B. and E. Espeland. 1998. Demography of natural and reintroduced populations of Acanthomintha duttonii. Demographic performance at Pulgas Ridge and Edgewood Park. Report prepared for Endangered Plant Program, California Department of Fish and Game. 25 pp.Pavlik, B. and E. Espeland. 1994. Creating new populations of Acanthomintha duttonii, an endangered serpentinite annual in northern California. Madrono, 45:1, pp. 31-39.

Steeck, D. M. 1995. Reproductive biology of a rare California annual, Acanthomintha duttonii, and its congener Acanthomintha obovata ssp. cordata. M.S. thesis, University of California, Davis, 42 pp.

USFWS. 1998. Recovery plan for serpentine soil species of the San Francisco Bay Area. Portland, OR. 330+ pp.

Picture Credits

Stuart B. Weiss