Verbena hastata (American vervain, blue vervain or swamp verbena) is a flowering plant in the vervain family, Verbenaceae. It is a herb with opposite, simple leaves which have double-serrate margins, borne on stiffly erect, branching square stems. The purple flowers appear in summer. This is a common plant that occurs across North America. It is hardy and drought resistant.
This species is a member of the diploid North American vervains which have 14 chromosomes altogether. Hybridization seems to have played some role in its evolution, presumably between some member of a group including the white vervain (V. urticifolia), V. lasiostachys or V. menthifolia, and V. orcuttiana or a related species.
In the recent evolutionary past, there has been an incident of chloroplast transfer of one of the latter or the swamp verbena to the mock vervain Glandularia bipinnatifida which is a close relative of the genus Verbena. It is unknown by what mechanism this happened, but it is suspected that hybridization is not responsible.
It is a larval host to the common buckeye butterfly.