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 and Coahuila.[2] It occurs in coastal prairie, grassland, and mesquite shrubland habitat. 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).[3] 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.[4] It is not certain that the plant still exists in Mexico.[5] 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.[6]
This plant sometimes occurs alongside slender rush-pea (Hoffmannseggia tenella), another endangered species.[5]
