Phragmites australis is a cosmopolitan species with two distinct lineages in North America: introduced Phragmites australis subsp. australis and native Phragmites australis subsp. americanus. Until recently, phenological barriers (timing of flowering and anthesis) were thought to prevent interbreeding between native and introduced Phragmites. However, preliminary data on 17 Phragmites populations grown in a common garden show significant overlap in flowering phenology, providing ample opportunity for interbreeding. Furthermore, in the field, native and introduced populations occur within short distances from one another at nine of the sites from which our plant material was collected. At least two other common gardens with the same haplotypes, as well as widespread wild populations, exist at several different latitudes in the U.S., making Phragmites an ideal model system to infer the effects of climate change on the phenology of Phragmites and similar marsh grasses. Future research will quantify phenological traits in common garden and wild populations across latitudes in North America, thus providing information on the potential for increased intraspecific gene flow.