These masses are surrounded by non-serpentine rocks of the Franciscan Formation, including greywacke which some populations of Camissonia benitensis occur.
Its reddish stems bear plentiful linear leaves less than 1 centimeter long and minutely toothed along the edges.
Camissonia benitensis seeds at and near the soil surface germinate with the first rains in fall or winter (November 1 - January 31).
[4] The two species are virtually indistinguishable with the naked eye when plants are small (under 6 centimeters), as is common in their native habitat, where they occur together.
[3] C. strigulosa is the common Camissonia species found growing on nonserpentine alluvial stream and river deposits within the range of C. benitensis.
Based upon the original discovery of C. benitensis on a serpentine alluvial stream terrace adjacent to Clear Creek, it was long believed that its only habitat type consisted of serpentine alluvial terraces adjacent to perennial streams and rivers, a very rare type of habitat.
Populations found near Priest Valley in 2010 extended the previously known range 15 kilometers to the southeast (USFWS 2020).
Common features of the stream terrace are friable serpentine soils that are stable due to very low gradient or no slope and high percentage of coarser substrate particles including sand, gravel, and cobbles.
Dense woody vegetation (closed shrub gaps) appears to be detrimental to C. benitensis due to shading effects.
Some areas of geologic transition zone habitat have been partially invaded by invasive annual grasses such as Bromus madritensis.
Prescribed fire is utilized by the Bureau of Land Management to control Centaurea solstitialis which once threatened to invade stream terrace habitat occupied by San Benito evening primrose near the confluence of Clear Creek and San Benito River.
Some areas of geologic transition zone habitat on private land are grazed by cattle which like wildfire, is beneficial to C. benitensis by removing competing vegetation, particularly invasive annual grasses.
C. benitensis evening primrose is well-buffered from extinction by very large soil seed bank 100 - 1000 times that of the number of plants seen with a population in any given year (USFWS 2020).
Historic impacts to San Benito evening primrose have included logging and mining (gravel, magnesite, chromite, cinnabar, and chrysotile asbestos).
The relatively level stream terraces were historically favored by settlers for home sites and to stage industrial activities.
A proposed mineral withdrawal for the New Idria serpentine mass will greatly limit any future mining in the area.
[2] The Clear Creek Management Area was under a temporary Emergency Closure instituted on May 1, 2008[2] when the Environmental Protection Agency reported that natural occurrences of chrysotile asbestos in the soil was above safe limits for people.
Discovery of the geologic transition zone habitat and numerous additional populations has greatly improved the outlook for C. benitensis.