By MARK BERNINGER and MARK DODERO

The City of San Diego is home to some of the most rare and special plants in the Southern California region. One of those rare plants is San Diego Thornmint (Acanthomintha Ilicifolia).

The city started monitoring these plants more that 20 years ago as part of a groundbreaking Rare Plant Monitoring Program, which is part of the Multiple Species Conservation Program (MSCP), enacted in 1997 to preserve habitat and help conserve the region’s spectacular biodiversity.

The Mission Trails Regional Park (MTRP) Foundation has been a critical partner in the MSCP, and they partnered with the city once again to help improve habitat and increase the park’s populations of San Diego Thornmint. The MTRP Foundation applied and was awarded a grant from the TransNet Program at SANDAG, and these funds allowed the Foundation to engage local rare plant expert Mark Dodero and RECON Environmental to expand the park’s San Diego Thornmint populations. After three years, the project reached the end of grant funding and it was a smashing success! The population of plants that once numbered under 100 individuals in 2017 blossomed to over 800 plants in spring of 2020. This is great news for this population at Mission Trails, but the work is far from over. Preserving each population and their genetic diversity is critical for a robust regional conservation effort.

The partnership that the MTRP Foundation has with the City of San Diego is a great example of the collaboration it will take to bring San Diego Thornmint and other rare and endemic plants back from the brink of extinction. Sharing the story of this success will hopefully lead to more opportunities to develop secure funding to expand and enhance conservation efforts across Mission Trails and, in doing so, boost the resiliency of the rare plants across the entire Southern California region.

Overview of project

San Diego Thornmint is found only in San Diego County in the United States and in northwestern Baja California. Populations of San Diego Thornmint are declining throughout San Diego County, and are threatened by pressures from urban development, habitat disturbance, and invasion of non-native species with as many as one-third of historical occurrences believed to have been lost. Thornmint typically grows in open areas with little competition with shrubs and other plant species.

The initial focus San Diego Thornmint Expansion Project in 2017 was to get the non-native weeds under control so that the Thornmint plants would have less competition for light and water. Also, during that first year, the San Diego Zoo Institute for Conservation Research’s Native Seed Bank began growing out additional Thornmint plants to produce as many seeds as possible. Prior to any seeding efforts, monitoring of the plant population in the spring of 2018 revealed the presence of 43 Thornmint plants in the natural population.

In the second year of the project, the weed control efforts continued and the first lot of bulked Thornmint seed was provided for dispersal. Fenced cages were constructed at the site to protect the seedlings from rabbits and rodents. The seeds were dispersed a few days prior to heavy rainfall, grew well and flowered in the spring. A total of 374 Thornmint flowered and set seeds successfully. Of those 374 plants 319 plants were found in the natural population and 55 plants were found in the seeded plots. Each year of the project, weeding reduced the cover of non-native species and this led to an increase in flowering native species.

In the project’s final year, additional Thornmint seeds were produced by the San Diego Zoo, and seed collected by the project biologists from the seeded and natural population were dispersed throughout the area. Because of dry conditions, the project biologist and maintenance crew gave supplemental water to the seedlings. This supplemental water allowed the seedlings an opportunity to survive until natural rainfall began again, enabling the plants to grow and successfully flower in the spring of 2020. A total of 846 Thornmint plants grew at the site in 2020. This represented an increase of over 800 plants from the beginning of the project.

Through partnerships between public agencies such as the City of San Diego and SANDAG and private groups like the Mission Trails Regional Park Foundation and San Diego Zoo, good conservation outcomes are possible.

— Mark Berninger is Natural Resources Manager for the City of San Diego and Mark W. Dodero is Senior Restoration Specialist at RECON Environmental.