Ambrosia cheiranthifolia
Conservation status
Imperiled  (NatureServe)
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Clade:	Asterids
Order:	Asterales
Family:	Asteraceae
Genus:	Ambrosia
Species:	A. cheiranthifolia
Binomial name
Ambrosia cheiranthifolia A.Gray

Ambrosia cheiranthifolia is a rare species of flowering plant known by the common names South Texas ambrosia and Rio Grande ragweed. It is native to the coast of South Texas^[1] and the Mexican states of Tamaulipas^[2] and Coahuila.^[3] It occurs in coastal prairie, grassland, and mesquite shrubland habitat.^[2] It has declined because its native habitat has been cleared for development, with remaining open savanna invaded by non-native grasses such as buffelgrass (Cenchrus ciliaris).^[4] Today there are perhaps 20 populations remaining, but some of these may have very few genetic individuals because the species is clonal, with many cloned plants attached by one rhizome.^[5] It is not certain that the plant still exists in Mexico.^[6] This is a federally listed endangered species of the United States.

Ambrosia cheiranthifolia is a rhizomatous perennial herb growing erect to a maximum height around 40 centimeters. Several clones usually grow in a dense patch. The stems and herbage are silvery green with a coating of rough gray hairs. The oblong leaves are 3 to 7 centimeters long and oppositely arranged on the lower plant but alternate on the upper stems. The inflorescence contains staminate flower heads in clusters with a few pistillate heads in leaf axils below the clusters.^[2]

This plant sometimes occurs alongside slender rush-pea (Hoffmannseggia tenella), another endangered species.^[6]