Bluebunch wheatgrass (BBWG; Pseudoroegneria spicata [Pursh] À. Löve) and Snake River wheatgrass (SRWG; Elymus wawawaiensis J. Carlson & Barkw.) are perennial cool-season grasses commonly used in rangeland restoration in the Intermountain West. The annual downy brome (Bromus tectorum L.) is widespread in the region, compromising perennial-grass establishment. We conducted two winter and one summer greenhouse target-neighbor trials to evaluate single-seedling performance of perennials grown with or without this annual grass neighbor (AGN). Our hypotheses were 1) an AGN impacts perennial wheatgrass seedlings' biomass and water-relations traits, 2) BBWG and SRWG differentially display traits when grown with and without an AGN, and 3) older wheatgrass cultivars differ from newer experimental populations for these traits. The AGN reduced shoot dry matter (DM) 28.2–33.1% and leaf area 32.5–35.9% across trials, but inconsistent differences for other traits suggested different drivers were operating among trials. In both winter trials, low humidity likely drove high vapor pressure deficits, leading to reduced water availability, with xylem pressure potential data suggesting greater water reduction in the third (winter) trial. In the second (summer) trial, heat rather than vapor pressure deficit was the likely driver. Both winter trials displayed higher shoot DM for BBWG under both AGN treatments, but in summer when the AGN was present, SRWG exceeded BBWG. In the summer trial, Goldar BBWG was sensitive to the combination of reduced water and heat. SRWG exhibited “faster” traits facilitating growth, while BBWG displayed “slower” traits conserving acquired resources. This may partially explain why SRWG's natural distribution is mostly restricted to rangelands with high soil fertility (e.g., the Palouse Prairie) while BBWG's distribution is more widespread. Newer experimental populations of both species often produced less shoot DM than older cultivars, suggesting the experimentals feature a more conservative growth strategy.