Abstract Hairy vetch is a promising legume cover crop ( Vicia villosa Roth) for the northern United States. Based on evidence from molecular markers, multi‐site evaluations, and morphological observations, a distinct second species exists within US hairy vetch germplasm, referred to hereafter as smooth vetch ( Vicia varia Host). Morphologically, hairy vetch is highly variable, but this study found statistically significant differences between smooth and hairy vetch in visual pubescence scores, plant maturity, and calyx lobe lengths. We used a panel of single sequence repeat (SSR) markers to assign cultivars and breeding materials to the two species and found that many commercial cultivars are smooth vetch. Interestingly, the SSR panel indicated that woollypod vetch ( Vicia dasycarpa Ten.) is a subpopulation of hairy vetch. Based on an elastic net model trained with multi‐site trial results from >35 site‐years, smooth vetch is not winter hardy in the northern United States but has superior performance relative to hairy vetch in the south‐central and Pacific Northwest United States. Specifically, smooth vetch has greater performance in environments with mild winters, cool spring temperatures, or low clay soils. Because of these differences in adaptation, differentiating these species will greatly improve agronomic outcomes and accelerate ongoing cover crop breeding progress.