It is a widespread genus with most species occurring in the tropical Americas,[2][3] and others in Asia, Africa, and Australia.

The inflorescence is a spike or a rounded head occurring in the leaf axils or the ends of branches.

[1] As of April 2024, Plants of the World Online accepts the following species:[1] Many species have been reported as noxious weeds, including A. angustifolia, A. caracasana, A. denticulata, A. nana, A. nodiflora, A. paronychioides, A. philoxeroides, A. sessilis, A. tenella, and A.

[5] The most important species is alligator weed (A. philoxeroides), a South American aquatic plant that has spread to other continents.

[5] It is a weed of many kinds of agricultural crops, it is an invasive species that degrades native habitat, and its dense mats of vegetation clog waterways, slowing shipping and increasing flooding.