South Texas Ambrosia (Ambrosia cheiranthifolia)

Photograph of the South Texas Ambrosia


Other Names
South Texas Ragweed, Rio Grande Ragweed
Texas Status
U.S. Status
Endangered, Listed 8/24/1994
A member of the aster family, South Texas ambrosia is an erect, silvery to grayish-green, perennial, herbaceous plant, 4 to 12 inches in height. Its simple leaves, about 3 inches long and 1.5 inches wide, are usually opposite on the lower portion of the plant and alternate above. Male and female flowers are separate but occur on the same plant. Male flowers are in heads arranged along a terminal, elongated stem. Flower stalks contain 10-20 small, yellowish, bud-like flowers, about 1/4 inch across and shaped like hanging bowls. Female flowers are in small clusters at the leaf bases below the male flowering stalks.
Life History
South Texas ambrosia blooms in late summer and fall, but its flowers are not showy and may be missed by the casual observer. It spreads through rhizomes (underground stems), and a single individual plant may be represented by hundreds of stems forming close-spaced colonies. It may occur in association with slender rush-pea, which is also federally-listed as endangered.

Associated native grasses found at the existing sites include Texas grama, buffalograss, Texas wintergrass, and tobosa. Native woody species found scattered throughout the existing sites include mesquite, huisache, huisachillo, brasil, granjeno, and lotebush. While south Texas ambrosia does not appear to survive continual plowing, sporadic disturbance may enhance its growth and spread. Studies are currently addressing the effect of mowing on the species.

Loss of habitat has led to the decline of this species. Conversion of habitat to agricultural fields and urban areas has limited the amount of habitat available for colonization. In addition, introduced species such as buffelgrass and King Ranch bluestem compete with this and other natives of the coastal prairie. Invasion of grasslands by shrub and tree species also contributes to loss of available habitat, although the species does occur among scattered woody plants. Disturbance associated with activities occurring along road right-of-ways where the species is found may also be detrimental.
South Texas ambrosia occurs in open grasslands or savannas on soils varying from clay loams to sandy loams.
Historically, South Texas ambrosia was known from Cameron, Jim Wells, Kleberg, and Nueces counties in South Texas, and the state of Tamaulipas in Mexico. Today, the species occurs at six locations in Nueces and Kleberg counties. The current status of any populations in Mexico is unknown.
South Texas ambrosia was listed as endangered by the U.S. Fish and Wildlife Service in August 1994. Landowners and managers can help conservation efforts by learning to recognize this plant and managing the sites to maintain diverse native rangeland plant communities. Mechanical brush management and herbicide use should be carefully planned to avoid damaging impacts to colonies of south Texas ambrosia.

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