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California Native Plant Society

Santa Clara Valley Chapter

Seeds of Hope for San Mateo Thornmint

San Mateo thornmint

Edgewood Park in San Mateo County is a very special place, as shown by over 20 years of preservation and restoration work by our chapter.  But did you know there’s a plant found only at Edgewood?  In fact, discovery of the federally endangered San Mateo thornmint (Acanthomintha duttonii) helped save Edgewood from becoming a golf course, providing critical evidence the site was worth preserving.


Formerly found on San Francisco Water District property and in Emerald Hills, this diminutive annual mint now occurs only at Edgewood, and sadly is in steep decline.  An estimated 53,000 plants were observed in 1998, but in 2008 just 249 were counted.  The primary culprit?  Invasion by non-native Italian rye grass (Lolium multiflorum).

This dire situation spurred biologists from Creekside Center for Earth Observation (CCEO) to begin an intensive propagation and restoration effort, funded by a Preventing Extinction grant from the U.S. Fish and Wildlife Service.  And CNPS volunteer weeders have been providing “boots on the ground” as needed to help with the effort, as well as pushing back other looming invaders at the periphery of the habitat like hedgedog dogtail grass (Cynosurus echinatus) and yellow star-thistle (Centaurea solstitialis).

These efforts reached a major milestone in early November, when CCEO scientists, park staff, and volunteers from Friends of Edgewood and CNPS collaborated to plant 12,500 tiny but precious thornmint seeds near the existing population.  Seed came from plants grown at Berkeley’s UC Botanical Garden (using seed collected years ago from Edgewood), and was planted in test plots with various treatments (scraping, hand weeding, control) to determine optimal restoration techniques for the site.  By late November, after 3 weeks of careful hand watering, tiny little thornmint seedlings were observed!

Restoration and seeding at Edgewood will continue in coming years, and reintroduction at other suitable sites near Edgewood may be attempted as well.  And though San Mateo thornmint is too specialized to ever become widespread in our area, it’s hoped these efforts will at least ensure it remains forever part of the wildflower heritage we hand to future generations.  Hats off to the dedicated team at CCEO, as well as to our chapter volunteers!

PS: While we’d love to share this project up close, we ask you to respect this rare and delicate plant by staying on the trails at Edgewood.

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