Ambrosia pumila

Ambrosia pumila is a rare species of herbaceous perennial plant known by the common names San Diego ragweed and San Diego ambrosia. It is native to far southern California, Baja California, and Baja California Sur. It grows in floodplains and open grasslands in proximity to wetland areas.[2][3][4][5]

Ambrosia pumila

Critically Imperiled  (NatureServe)
Scientific classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Angiosperms
Clade: Eudicots
Clade: Asterids
Order: Asterales
Family: Asteraceae
Genus: Ambrosia
Species:
A. pumila
Binomial name
Ambrosia pumila
Synonyms[1]

Franseria pumila Nutt.

Distribution

It is adapted to dry habitat, but only on upper floodplain fringes, or adjoining depressions containing vernal pools or similar structures.[6] It is a plant of open habitat and is not tolerant of heavy shade.[6]

Today it is known from 19 populations. Fourteen of them are in San Diego County, two exist in Riverside County, and there are three south of the border in Baja California and Baja California Sur in Mexico.[7] Most of its native habitat has been consumed by urbanization and development. It is also threatened by agriculture. This is a federally listed endangered species of the United States.Calflora taxon report, University of California: Ambrosia pumila (San Diego ambrosia, San Diego ragweed)

Description

Ambrosia pumila is a hairy perennial herb not exceeding 0.5 metres (1.6 ft) 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 is tipped with staminate (male) flower heads above several larger pistillate (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.[6] The plant reproduces vegetatively, sending up new sprouts from an elongated rhizome system.

Conservation

Critical habitat was declared for the species in 2010, listing areas that should be protected for the survival of the plant.[6] Protected areas containing this plant include sections of land in the cities of Temecula and Oceanside, near Fallbrook and Lake Hodges, and within the bounds of Mission Trails Regional Park and the San Diego National Wildlife Refuge.[6]

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.[8][9]

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References
  1. The Plant List Ambrosia pumila (Nutt.) A.Gray
  2. Flora of North America Vol. 21 Page 18 Ambrosia pumila (Nuttall) A. Gray, Proc. Amer. Acad. Arts. 17: 217. 1882.
  3. Tropicos, specimen listing for Ambrosia pumila (Nutt.) A. Gray
  4. Shreve, F. & I. L. Wiggins. 1964. Vegetation and Flora of the Sonoran Desert 2 vols. Stanford University Press, Stanford
  5. Biota of North America Program 2014 county distribution map
  6. USFWS. Final Rule Designating Critical Habitat for Ambrosia pumila (San Diego ambrosia). Federal Register November 30, 2010.
  7. The Nature Conservancy
  8. Kelly, M., et al. (2007). Spraying over the top of Ambrosia pumila, a federally listed species, to control invasive weeds. Archived 2011-09-30 at the Wayback Machine Proceedings California Invasive Plant Council Symposium 2007.
  9. Maher, E. and E. Stanton. (2006). Response of the endangered San Diego ambrosia (Ambrosia pumila) to removal of competition from non-native plants. Archived 2011-09-30 at the Wayback Machine Proceedings California Invasive Plant Council Symposium 2006.

Further reading
  • McGlaughlin, M. E. and E. A. Friar. (2007). Clonality in the endangered Ambrosia pumila (Asteraceae) inferred from RAPD markers; Implications for conservation and management. Conservation Genetics 8:2 319.
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