Sampling
A set of 146 coarse-textured P. vaginatum , 18 fine-texturedP. vaginatum and 10 P. distichum plant samples was collected along the Atlantic and Gulf coasts of the southeastern United States, from North Carolina to Texas, during June and July of 2016 and 2017 (Fig. 1A; Table S1). Collection habitats included beaches, salt marshes, brackish estuaries and freshwater rivers and lakes. To reduce the chance of repeatedly collecting representatives of the same clone, localities were separated by at least 1.5 km. One to three samples were collected in each location. When multiple samples were collected from a single locality, they were separated by at least 100 m to minimize replicated sampling of a single clone. Samples were harvested as stolons which were wrapped in a damp paper towel and then placed in a plastic bag for no longer than one week before planting. Stolons were propagated in the Donald Danforth Plant Science Center greenhouses. In addition to the wild-collected material, a set of 40 samples, representing 35P. vaginatum (two of which are coarse-textured) and five P. distichum , were obtained from the USDA National Genetic Resources Program. Additionally, dried tissue for DNA extraction, representing four P. vaginatum accessions and one P. distichumaccession, were acquired from Uruguay from Dr. Pablo Speranza (Universidad de la República de Uruguay).