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NATURAL HISTORY

SAN DIEGO AMBROSIA } Ambrosia pumila
FAMILY: Asteraceae


DESCRIPTION: The San Diego ambrosia is a low, creeping perennial herb that spreads by means of slender, branched, underground rhizome-like roots. Stems are usually between two and 12 inches tall and are covered with short hairs. The plant’s small leaves are blue-gray in color, and it has clusters of tiny, yellow flowers summer through fall.

HABITAT: San Diego ambrosia is primarily restricted to flat or gently sloping grasslands and upper terraces of rivers and drainages. The species may also be found in openings in coastal sage scrub, adjacent to vernal pools, and in disturbed sites such as those near roadways. San Diego ambrosia generally occurs at low elevations — less than 1,600 feet in the case of the Riverside populations and less than 600 feet in San Diego County.

RANGE: Populations of San Diego ambrosia occur in San Diego and Riverside counties in California and in the northern state of Baja California, Mexico.

BREEDING: San Diego ambrosia often reproduces clonally by utilizing a branching, underground root structure. The plants also have the ability to self-pollinate or to be wind pollinated, and they have both male (stamen) and female (pistil) parts on the same stem.

THREATS: San Diego ambrosia is threatened primarily by urban sprawl. Other threats include agricultural expansion, livestock grazing, off-road vehicles, pesticide spraying, road construction, and the introduction of exotic species.

POPULATION TREND: Historical records indicate that the San Diego ambrosia was once found within most major drainages in California’s San Diego County. Today, fewer than 15 viable populations of this species remain, down from 49 as recently as 1999. Of these, the long-term viability of seven populations is questionable, and another consists of only a single plant. Three populations persevere in Mexico, all of which are threatened by urban and agricultural development.

Photo © Jim Rocks