''Ambrosia pumila'' is a rare species of

herbaceous Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition of t ...

perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

plant known by the common names San Diego ragweed and San Diego ambrosia. It is native to far southern

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

Baja California Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Mex ...

, and

Baja California Sur Baja California Sur (; 'South Lower California'), officially the Free and Sovereign State of Baja California Sur ( es, Estado Libre y Soberano de Baja California Sur), is the least populated state and the 31st admitted state of the 32 federal ent ...

. It grows in

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

s and open

grasslands A grassland is an area where the vegetation is dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur natural ...

in proximity to

wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...

areas.

Distribution

It is adapted to dry habitat, but only on upper floodplain fringes, or adjoining depressions containing

vernal pool Vernal pools, also called vernal ponds or ephemeral pools, are seasonal pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the safe ...

s or similar structures.USFWS
Final Rule Designating Critical Habitat for ''Ambrosia pumila'' (San Diego ambrosia).
''Federal Register'' November 30, 2010. It is a plant of open habitat and is not tolerant of heavy shade. Today it is known from 19 populations. Fourteen of them are in

San Diego County San Diego County (), officially the County of San Diego, is a county in the southwestern corner of the U.S. state of California. As of the 2020 census, the population was 3,298,634, making it California's second-most populous county and the fi ...

, two exist in

Riverside County Riverside County is a county located in the southern portion of the U.S. state of California. As of the 2020 census, the population was 2,418,185, making it the fourth-most populous county in California and the 10th-most populous in the Unit ...

, and there are three south of the border in

and

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

.The Nature Conservancy
/ref> Most of its native habitat has been consumed by urbanization and development. It is also threatened by

agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...

. This is a federally listed

endangered species An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and inv ...

of the United States.

Description

''Ambrosia pumila'' is a hairy perennial herb not exceeding in height. The leaves are gray-green and fuzzy and divided into several subdivided segments. They are up to 13 centimeters long, not counting the winged petioles. The

inflorescence An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Morphologically, it is the modified part of the shoot of seed plants where flowers are formed o ...

is tipped with

staminate The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filam ...

(male)

flower heads A pseudanthium (Greek for "false flower"; ) is an inflorescence that resembles a flower. The word is sometimes used for other structures that are neither a true flower nor a true inflorescence. Examples of pseudanthia include flower heads, compos ...

above several larger

pistillate Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) ''pistils'' ...

(female) heads. The pistillate heads each yield usually one fruit, which is a fuzzy burr only a few millimeters wide with short, soft spines. The plant rarely produces seeds. The plant reproduces vegetatively, sending up new sprouts from an elongated

rhizome In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...

system.

Conservation

Critical habitat was declared for the species in 2010, listing areas that should be protected for the survival of the plant. Protected areas containing this plant include sections of land in the cities of

Temecula Temecula (; es, Temécula, ; Luiseño: ''Temeekunga'') is a city in southwestern Riverside County, California, United States. The city had a population of 110,003 as of the 2020 census and was incorporated on December 1, 1989. The city is a t ...

and Oceanside, near

Fallbrook Fallbrook is a CDP in northern San Diego County, California. Fallbrook had a population of 30,534 at the 2010 census, up from 29,100 at the 2000 census. Fallbrook's downtown is not on a major highway route. It is west of Interstate 15 or n ...

and

Lake Hodges Lake Hodges is a lake and reservoir located within the city limits of San Diego, California. It is about north of Downtown San Diego, just north of the Rancho Bernardo community, and just south of the city's border with Escondido. When full, th ...

, and within the bounds of

Mission Trails Regional Park Mission Trails Regional Park is a open space preserve within the city of San Diego, California, established in 1974. It is the sixth-largest municipally owned park in the United States, and the largest in California. Description The park cons ...

and the San Diego National Wildlife Refuge. One threat to the species is the invasion of non-native weeds, which tower over the plant and shade it out. It is challenging to remove the dense stands of weeds without injuring the rare plant.Kelly, M., et al. (2007)
Spraying over the top of ''Ambrosia pumila'', a federally listed species, to control invasive weeds.
Proceedings California Invasive Plant Council Symposium 2007.Maher, E. and E. Stanton. (2006)
Response of the endangered San Diego ambrosia (''Ambrosia pumila'') to removal of competition from non-native plants.
Proceedings California Invasive Plant Council Symposium 2006.

References

External links

Jepson Manual Treatment — ''Ambrosia pumila''''Ambrosia pumila'' Calphotos Photo gallery,University of California

Calflora taxon report, University of California: ''Ambrosia pumila'' (San Diego ambrosia, San Diego ragweed)
{{- pumila Flora of California Flora of Baja California Flora of Baja California Sur Plants described in 1840 Natural history of the California chaparral and woodlands Natural history of Riverside County, California Natural history of San Diego County, California Critically endangered flora of California

Distribution

Description

Conservation

References

Further reading

External links