A rare plant that depends on wetlands for survival is now on the federal endangered species list, a designation that environmentalists say will boost efforts to protect the last free-flowing river in the desert Southwest.

The U.S. Fish and Wildlife Service published the decision Friday in the federal register to list the Arizona eryngo as endangered and set aside nearly 13 acres (5 hectares) in southern Arizona as critical habitat.

The decision comes years after environmentalists petitioned and then sued to gain protection for the plant with cream-colored spherical flower heads. Only two populations are known in Arizona near Tucson and in the San Pedro Riparian National Conservation Area.