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On a recent sunny morning, a small group of conservationists heads into battle in the hills above Redwood City. They don an armor of bug spray and gardening gloves and walk together into their battlefield; a small, off-trail section of land in the Edgewood Park and Natural Preserve.
They are there to protect the San Mateo thornmint, an endangered mint species that naturally grows only in one place in the world: the serpentine vertisol soils of Edgewood Park. This rare combination of serpentine soil, known for its high levels of heavy metals, and vertisol, which is charactertized by high clay content. creates a unique and challenging environment for plant growth.
On this particular day, the group works to remove parasitic vines from the thornmint habitat.
For years, this fragrant herb with purple and white flowers was on the verge of extinction. Now, due in large part to ongoing efforts from local biologists and volunteers, the thornmint is not only making a comeback, it is also starting to take root elsewhere. In January, scientists observed that plants they seeded in Marin County – 60 miles north of the thornmint’s historical habitat – have begun to grow, gaining a foothold in a new environment. As of May, some had even begun to flower.
“I feel like we definitely saved it from extinction,” said Stuart Weiss, founder and chief scientist of Creekside Science, the freelance conservation organization that has been spearheading the thornmint’s conservation efforts since 2008. “We’ve given it a much better chance, but it’s like a lot of species: It’s always going to be somewhat conservation dependent.”
What’s so special about thornmint?
So what’s all the fuss about this little local plant? The thornmint isn’t a so-called umbrella species, which has sprawling effects on the ecosystem. It can’t be grown in backyard gardens, and humans don’t use it for food or for drugs. What thornmint can do, conservationists point out, is support local wildlife within its unique ecosystem, even if that area only covers a narrow strip of land stretching about six miles.
Christal Niederer, Creekside’s senior biologist and leader of the thornmint project, shared this analogy: “You’re sitting on an airplane, and you see a rivet pop off of the wing. You say, ‘Oh, well it’s just one rivet,’ and then another one falls off, and then another one, and at some point, the plane’s not going to be safe,” she said.
Thornmint shown in bloom (left) compared to what it looks like in June when the plant is dead for the season. Photo on left courtesy Creekside Science. Photo on right by Grace Gormley.
Maybe somewhere in (thornmint) biochemistry is some drug, but we’ll never know if we lose it.
Stuart Weiss, founder and chief scientist, Creekside science
The thornmint is like a rivet. It plays a role in how the earth functions. It can grow in serpentine grasslands that are unsuitable for most other plants and contributes to the biodiversity of that unique ecosystem.
Weiss emphasizes that we don’t know everything about thornmint yet, and a broader purpose could be revealed.
“Maybe somewhere in (thornmint) biochemistry is some drug, but we’ll never know if we lose it,” Weiss said. “It’s like burning a library before we know how to read the books.”
More importantly, Weiss said, the thornmint is “a beautiful little plant.” It is a local treasure that grows only in our Bay Area soil, and it survived four and a half billion years since the origin of life.
“It deserves a chance to keep going, because we inadvertently almost drove it to extinction. Who cares about a purpose?” he said.
The comeback
When Weiss and his organization stepped up to help save the thornmint in 2008, its population had dropped to only 249 plants, in large part to non-native invasive plants like Italian rye grass. Thornmint used to grow in a broader area, including in the Emerald Hills neighborhood in unincorporated San Mateo County, but the population started shrinking decades ago. The state of California first listed it as endangered in 1982.
“Always out of the corner of my eyes, I’d been thinking about the thornmint,” Weiss said. “It was like, ‘oh, we’d better start doing something!’”
In fall of 2009, volunteers planted almost 13,000 seeds around the thornmint’s natural habitat in Edgewood, thanks to two grants from the U.S. Fish and Wildlife Service , “Preventing Extinction,” and “Partners for Fish and Wildlife.” Since then, funding for restoration work has come from a variety of sources, including federal grants, local government funding from San Mateo County Parks and the San Mateo County Parks Foundation, crowdfunding efforts, and funding from environmental organizations like Friends of Edgewood.
It’s really exciting to think (we may have) prevented an extinction, just by trying.”
Christal Niederer, senior biologist, Creekside science
Over the years with more planting, weeding and monitoring, the plant began to slowly grow back. Today, the little thornmint and its purple blooms finally grace the hills of Edgewood again. (Park visitors won’t likely have an opportunity to see the plants up close, as the specific growing locations are often kept confidential to protect the species and ongoing restoration efforts.)
Beyond making a comeback in its original habitat, the plant also has been successfully introduced to similar soil outside of its historical habitat. There have been some plants growing at Pulgas Ridge just north of Edgewood since 2018, and a new population is taking root this year on Marin’s Ring Mountain.
“Having more populations in different environments gives you resiliency against fluctuations in weather or some catastrophe,” Weiss said.
The total population in 2024 was about 38,000, which conservationists “feel pretty good about,” Weiss said.
Ongoing efforts
Though much has been done to save the thornmint, there is still more to do. Weiss estimates that the thornmint requires at least five conservation trips per year for various tasks like counting, planting or weeding.
That’s where the group of conservationists that set out on a recent sunny morning in late June comes in. The thornmint are dead for the season, browned by the summer sun, but the group’s conservation work doesn’t stop. Niederer joins three volunteers to remove parasitic dodder plants from the thornmint’s habitat.
Dodder is orange, not green, because instead of photosynthesis, it gets energy from wrapping around other plants and sucking their energy. The volunteers remove the dodder by carefully finding affected plants and weeding them out.
It’s very painstaking, tedious work. The romance of field work quickly wears off when you’re down there on the ground.
Stuart Weiss, founder and chief scientist, Creekside science
“It’s very painstaking, tedious work,” Weiss said. ”The romance of field work quickly wears off when you’re down there on the ground.”
Weiss said volunteers have been in the field – literally – throughout the process, thanks to Friends of Edgewood, a community group that has assisted with the thornmint’s conservation efforts at the park since the start of the project. The group organizes volunteers and fundraises to protect the park and aided Creekside Science as they seeded and monitored plants all around Edgewood. Even now, as the plant has made a comeback, volunteers are still in the field making certain the thornmint thrives.
Sandy Bernhard, a member of the board of Friends of Edgewood and one of the volunteers on the recent morning mission, has been working to bring back thornmint since 2009. She said working with the team to restore the plant makes her feel inspired.
The thornmint, though it will need continued support, tells us that there is hope, according to Niederer.
“We hear so much negative about the environment, the planet, the damage we’re doing, and it is true, and it is real, but we can also do good things,” she said. “It’s really exciting to think (we may have) prevented an extinction, just by trying.”
It’s sad that the author of the article didn’t include the fact that the decades long fight for preservation of Edgewood Nat’l Preserve actually began with this plant! Susan Sommers was, for the first decade of this effort, alone in her quixotic effort. She was not a botanist, but her grandmother was, and her grandmother had discovered the thornmint which was named after her, Acanthomintha duttonii. When Susan learned that this one small parcel of land on which this plant grows was to be turned into a gold course, she thought “over my dead body!” and set out to learn all the native plants that grew at Edgewood to use the extraordinarily rich biodiversity as an argument for protection of the land. Battles were won and lost, repeatedly, with her making presentations that moved or were ignored by bodies of decision makers. The land was to become a community college campus. And then, for a second time, a golf course. Susan was eventually joined by others, eventually many runners, hikers and nature lovers as well as botanists who could continue and expand her advocacy efforts. Not until the early nineties did we finally prevail in permanently protecting the land on which this tiny plant, and over 500 native species, including other rare and endangered ones, survive as a treasure to be visited and enjoyed in perpetuity. Sommers deserves credit as the original champion. It’s a shame that critical first chapter of the story was omitted. But I honor her and for the 24 yrs. I led wildflower hikes at Edgewood I always gave her the credit for the magnificently biodiverse property to which I was introducing visitors. Here’s to you, Susan!