San Mateo thornmint

The range of this species is sharply limited within a portion of central San Mateo County on the eastern lower slopes of the Santa Cruz Mountains. The single remaining large population, in Edgewood County Park, is a relict of a more extensive colony damaged by off-road motor-vehicle use. There is an introduced population at Pulgas Ridge Open Space Preserve, and native fragments along the lower slopes above Crystal Springs Reservoir in a six-mile-long (10 km) fragmented strip that includes the Edgewood colony.

This species is only known to grow on serpentine soils in grassland communities that are generally species-rich for serpentine soils; the area they are growing in on the San Francisco Peninsula also contains sloping chaparral. Other native species that grow in the same area include Nassella pulchra, Delphinium hesperium, and Hemizonia congesta var. luzulifolia, plus the exotic Lolium multiflorum. Specific plant communities where this species is found are chaparral as well as foothill or valley grasslands. San Mateo thornmint populations occupy slopes or flatland with deep, heavy clay soil inclusions. In general, serpentine soils normally provide an inhospitable environment for most plants. Several factors contribute to serpentine soils being inhospitable to plant growth including a low calcium-magnesium ratio, lack of essential nutrients namely nitrogen, potassium, and phosphorus, and high concentrations of heavy metals. Many species that have evolved to grow on serpentine soils have are not adapted to compete with other plants and do not survive well under competition in other soils that tend to have denser plant numbers; in serpentine soils plant densities are lower resulting in less direct competition for resources like light and water.