The number and identity of species in the North American genus Euthamia (Asteraceae, Astereae) have varied considerably among taxonomic treatments. Euthamia graminifolia (L.) Nutt. is often treated to broadly include plants from the northern and eastern United States and Canada, including the Atlantic and Gulf Coasts. Broad-leaved, largely glabrous plants from New Jersey to the Florida Panhandle have been inconsistently treated as E. graminifolia, E. graminifolia var. hirtipes (Fernald) C.E.S. Taylor & R.J. Taylor E. hirtipes (Fernald) Sieren , or a hybrid between E graminifolia and E. caroliniana (L.) Greene ex Porter & Britton . Similarly, plants from the Florida Panhandle to eastern Louisiana have been incorporated into either E. graminifolia or E. graminifolia var. hirtipes, with only Greene in 1902 recognizing southern plants as a distinct species, E. scabra Greene . To determine the identity and proper rank of these entities, morphological and phylogenetic analyses were performed to evaluate relationships within Euthamia. Plants from the Atlantic Coast are most morphologically similar to those from the Gulf Coast, which morphologically resemble E. gymnospermoides Greene, a largely midwestern prairie species. The Gulf Coast plants and E. gymnospermoides share similar DNA sequences while those of the Atlantic Coast plants are a more distantly related clade. Neither the Gulf Coast nor the Atlantic Coast plants’ ribosomal DNA sequences are highly polymorphic, indicating that they are not likely of hybrid origin. Occasional plants found within southernmost Alabama and the Florida Panhandle have polymorphic sequences and intermediate morphological features however, suggesting that putative hybrids exist between the Gulf Coast and Atlantic Coast plants. This study concludes that entities deserve species rank as E. scabra Greene for scabrous plants occurring in the central Gulf Coast region and E. hirtipes (Fernald) Sieren for the largely glabrous plants mostly along the Atlantic Coast. Ecological niche modeling indicates that precipitation, especially during the summer months, and soils featuring coarse fragments and elevated sand content, drive the distribution of these organisms, largely restricted to either side of the Apalachicola River that often serves as a distributional barrier.

Morphological data was collected by evaluating five parts of each feature measured (i.e., five leaves to determine leaf width and length for one specimen) and averaging the results (or taking the mean for features solely with integers, such as floret numbers).

Molecular data was acquired by Sanger Sequencing two loci - ITS (using Hoshi primers) and ETS (using 18S-ETS primers). The ".bipartitions" file refers to the RAxML maximum likelihood tree with fast bootstrap support values, and the ".nex.con.tre" file refers to the MrBayes output with posterior probabilities.

Coordinate data were extracted from specimens or augmented with field collections/documentation.