San Mateo Thornmint

San Mateo thornming
San Mateo Thornmint

Acanthomintha duttonii

By Christal Niederer

For decades this plant was known only from one small population at Edgewood, but after lots of help this plant is defying extinction!

Restoration for San Mateo thornmint continues to make huge strides. In 2023, we estimated ~63,000 individuals at six locations, including five at Edgewood Preserve. This is up from our project low of 249 in 2008.

A Critically Endangered Plant

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.

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.

For decades this plant was known only from Edgewood, and this one population has been in decline for years. Researchers Pavlik and Espeland 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 (Festuca perennis), possibly exacerbated by nitrogen deposition syndrome. Most plants were found in the few relatively open areas within the historic habitat. Creekside Science initiated a restoration project aimed at San Mateo thornmint recovery. A key goal agreed upon by Creekside Science and the U.S. Fish and Wildlife Service is to have at least five self-sustaining populations of more than 5,000 individuals.

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. Key actions completed to date include:

  • Experimental seeding. On November 6, 2009, 12,500 seeds were sown into a total of 25 1-meter scraped, hand weeded, and control plots at Edgewood. By May 2010, there were 3135 total thornmint, including 250 in the unseeded control area. Scraping and seeding was identified as the best treatment, and those treatments were scaled up in subsequent years.
  • 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 showed they are similar to the Original site. Additional potential introduction sites at Edgewood were also identified.
  • Habitat enhancement. San Mateo thornmint is easily outcompeted by nonnative annual grass and its associated thatch. Habitat is carefully mowed to decrease grass cover and increase bare ground the San Mateo thornmint needs to thrive.
  • Seed increase. The UC Berkeley Botanical Garden and the Creekside Science Conservation Nursery have produced over 500,000 seeds by planting seeds left over from an old experiment, supplemented with wild-collected seeds.
  • 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.
  • Population monitoring. San Mateo thornmint have been censused every year since 2007. The unseeded control area, which represents plants outside the scope of the restoration project, got as low as 28 individuals in 2017. While the Original site has fluctuated up and down even with seeding, new locations give hope to the recovery of the species.
  • Introductions. Three new sites at Edgewood have been seeded, as well as one at nearby Pulgas Ridge. In May 2018, we were pleased to report 19,185 plants at five occurrences, the highest number since this project began in 2008. This plant is defying extinction!
Staff and Volunteers Prepare Seeding Plots
Staff and Volunteers Prepare Seeding Plots.
© 2014 Lech Naumovich Photography
Seeding Thornmint Plots
Seeding Thornmint Plots
© 2016 Christal Niederer
Looking for Thornmint Seedlings
Looking for Thornmint Seedlings

Working Toward Delisting

We continue to be motivated by the USFWS Recovery Plan goal of five self-sustaining populations, and a new delisting goal of ten self-sustaining populations. Creekside Science is conducting two strategies toward eventually downlisting this species.

The first is considered an enhancement phase, where we scale up our success. The phase focuses on continued seed introduction and site management, where we aim to bring sites to the point where they can be self-sustaining.

The second strategy is to find additional introduction sites. Not all the existing sites appear to be likely candidates to be long-term, self-sustaining populations. Finding additional sites at Edgewood Preserve may be possible, but as a resiliency strategy, some introductions should be outside of the preserve to guard against stochastic events and other impacts. We intend to continue looking for new sites, including sites in Marin County such as Ring Mountain that have promising habitat.
With our help, the San Mateo thornmint is defying extinction!


We are grateful to the Friends of Edgewood and USFWS for continued funding of this project. We also appreciate our partners at San Mateo County Parks for mowing and financial support; as well as the San Mateo County Parks Foundation. We continue to thank our volunteers and staff from Yerba Bioadvocacy and California Native Plant Society; as well as California Department of Fish and Wildlife.

San Mateo Thornmint © JPuenelle