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Ambrosia pumila
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San Diego Ambrosia
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Ambrosia pumila

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About San Diego Ambrosia (Ambrosia pumila) Ambrosia pumila is a rare species in the Asteraceae (Sunflower) family known by the common names San Diego Ambrosia and San Diego Ragweed. It is native to a very limited area in far southern California and Baja California where it grows in a variety of habitats along the coastal strip, inland valleys and foothills at elevations below 2,000 ft. Due to its rarity it is included on CNPS list 1B.1 and is listed as Endangered by the federal government. This is a perennial herb not exceeding half a meter in height. The leaves are gray-green, fuzzy, and divided into numerous minute lobes to give a feathery appearance. They are up to 13 centimeters long, not counting the winged petioles. The flower cluster 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. However, it rarely produces viable seed, usually spreading by its rhizome, forming clonal groupings. It is adapted to dry habitat, but only on upper floodplain fringes, or adjoining depressions containing vernal pools or similar structures. It is a plant of open habitat and is not tolerant of heavy shade. If not given supplemental summer water it will become deciduous but will come back from the rhizome after winter rain. This is not an easy plant to find and infrequently used in residential gardens. It is important for restoration projects in areas of appropriate habitat, and it may be useful in gardens seeking to specialize in rare plants of southern California.
Plant Description
Plant Type
Perennial herb

Max. Height
1.6 ft (0.49 m)

Max. Width
3 ft (0.9 m)


Growth Rate
Fast, Moderate

Summer Semi-Deciduous

Flower Color
Green, Yellow

Flowering Season
Spring, Summer, Fall

Native Status
Natural Setting
Site Type
Occurs primarily on upper terraces of rivers and drainages as well as in open grasslands, openings in coastal sage scrub, and occasionally in areas adjacent to vernal pools; may also be found in disturbed sites such as fire fuel breaks and edges of dirt roadways.


Elevation ?
33' - 1960'

Annual Precip. ?
9.9" - 15.0"

Summer Precip. ?
0.20" - 0.98"

Coldest Month ?
46.7° F - 55.4° F

Hottest Month ?
71.0° F - 80.4° F

Humidity ?
1.64 vpd - 28.65 vpd

Soil Description
Tolerates a variety of soils

Fast, Medium

Cold Tolerance(° F)
Tolerates cold to 15° F

Companion Plants
In the wild it is associated with Saltgrass (Distichlis spicata), Mulefat (Baccharis salicifolia), Broom Baccharis (Baccharis sarathroides), California Buckwheat (Eriogonum fasciculatum), and Turkey-mullein (Croton Setiger)

Wildlife Attracted
Species in the Ambrosia genus are host plant for the Bordered Patch butterfly

Landscaping Information
Water Requirement ?
Low, Moderate - High
Extremely Low
Very Low
Moderate - High

Rarely Used

Max. Summer Water ?
No Summer Water, 1x/month
No Summer Water
Keep moist

Propagation ?
Propagation is by rhizome cutting, rarely from seed

Nursery Availability
Commonly Available

Other Names
Common Names
San Diego Ragweed, Dwarf Bur Ragweed

Sources include: Wikipedia. All text shown in the "About" section of these pages is available under the Creative Commons Attribution-ShareAlike License. Plant observation data provided by the participants of the California Consortia of Herbaria, Sunset information provided by Jepson Flora Project. Propogation from seed information provided by the Santa Barbara Botanical Garden from "Seed Propagation of Native California Plants" by Dara E. Emery. Sources of plant photos include CalPhotos, Wikimedia Commons, and independent plant photographers who have agreed to share their images with Calscape. Other general sources of information include Calflora, CNPS Manual of Vegetation Online, Jepson Flora Project, Las Pilitas, Theodore Payne, Tree of Life, The Xerces Society, and information provided by CNPS volunteer editors, with special thanks to Don Rideout. Climate data used in creation of plant range maps is from PRISM Climate Group, Oregon State University, using 30 year (1981-2010) annual "normals" at an 800 meter spatial resolution.

Links:   Jepson eFlora Taxon Page  CalPhotos  Wikipedia  Calflora

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