Rare Plant Updates
Mission Trails
Ambrosia pumila Management Plan
November 1, 1999

The City of San Diego, in accordance with the Multiple Species Conservation Plan (MSCP), is adopting a management plan for San Diego ambrosia (Ambrosia pumila) within Mission Trails Regional Park (MTRP). San Diego ambrosia is an MSCP-covered species which means that conservation and management of plants within Multi-Habitat Planning Areas (MHPA), such as MTRP, is required. The species is also currently under a 12-month review period by the U.S. Fish and Wildlife Service (USFWS) for listing of the species as endangered. The management plan focuses on MTRP because it supports the second largest population of this narrow endemic species. Both State and Federal agencies have identified MTRP as a critical conservation area. San Diego ambrosia is found in southern Riverside county, San Diego county, and northern Baja California. Within the species' range there are only 21 confirmed localities. This low-growing perennial herb is primarily found in open habitats such as native grasslands within floodplains. A few common plant associates are purple needlegrass (Nassella pulchra), graceful tarplant (Halopcarpha virgata), saltgrass (Distichlis spicata), and tarweed (Hemizonia fasciculata). San Diego ambrosia has rhizome-like roots that produce above ground shoots in the winter. Flowering begins in June and continues through October.

The primary goals of the management plan are to protect the existing populations of San Diego ambrosia within MTRP and to recommend park management tools which will lead the persistence and possible expansion of these populations. The plan focuses on protection of the species by directing future development projects within MTRP away from existing populations. Only in extreme cases will translocation of the species be recommended as a mitigation measure. Currently, there are projects in San Diego county that indicate translocation may be a viable alternative. If done properly translocation of salvaged plants and container grown stock may be a good method of expanding existing San Diego ambrosia populations thereby increasing the stability of the species.

It should be emphasized that translocation of this species should be done in conjunction with native grassland restoration and that restoration efforts be expanded to include enhancement of sensitive species that co-occur with San Diego ambrosia, such as variegated dudleya (Dudleya variegata), San Diego goldenstar (Muilla clevelandii), and Orcutt's brodiaea (Brodiaea orcuttii).This way, several sensitive species benefit from the protection and management of one species and the diversity of native grassland habitat is maintained. The management plan also identifies immediate threats to San Diego ambrosia populations which include competition with exotic plant species particularly non-native grasses. The species rarely thrives in areas dominated by non-native grasses such as wild oat (Avena barbata) because the annual thatch build-up may hinder production of new shoots. Local biologist and park planners recognize that a major threat to San Diego ambrosia is the increase in exotic plant species. An active exotic plant species control program which focuses on the removal of non-native grasses from areas that support this species may be necessary to help stabilize these populations.

-- Carrie Stewart, Rare Plant Chairperson