It is narrowly endemic to the sandstone coastal terraces of San Diego County and northwestern Baja California.

It is listed as endangered and is threatened by land development, invasive species and modifications to the natural fire regime.

crassifolia include the presence of small, markedly flattened fruits and the fact that young stems and leaves lack glandular hairs.

The inland occurrences are found near San Marcos extending down into Mira Mesa, and southeast to MCAS Miramar and Mission Trails Regional Park.

[2] The primary habitat where this shrub is found is the southern maritime chaparral, an ecological community characterized by the dominance of chamise and black sage, and associated with coastal fog from Carlsbad south to Point Loma.

Fish and Wildlife Service only estimates that around 2,700 acres of the southern maritime chaparral is intact, an 87% reduction of the community that supports the Del Mar manzanita.

There is generally no specialized seed dispersal mechanism, with the fruits falling close to the parent plant in late summer where they are eaten by rodents, foxes or coyotes.

On MCAS Miramar, the plant is found on the training areas and rifle and pistol range surface danger zone, along with development for military housing.

[2] In San Diego County, fuel modification, which is the breakup of vegetation to create defensible space, poses a threat to the species.

The motivation for fuel modification is caused by the desire of homeowners, who border the habitat, to defend their homes from possible fires.

Long-term suppression of natural fire cycles can cause plants to grow crowded and unhealthy, becoming overcrowded with dead branches, fungi, and lichen.

Events like the Cedar Fire have had positive impacts on populations, with plants showing vigorous growth and robust branching after they resprouted from their burls.

These pose a significant threat to the plant, as they may burn seedlings and burls that have not had sufficient time between fires to build up nutrient reserves or effective lignotuber systems.

Non-native grasses like Ehrharta calycina cover the ground like a mat and spread quickly, enabling surface fires.

The invasive Eucalyptus species grow considerably tall, crowding out native plants, but also deposit large amounts of leaf litter on the surface of the soil.

In addition, compounds from Eucalyptus have allelopathic effects on other plants, inhibiting growth, germination, and nitrogen fixation of other species.