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Methods that allow rapid and robust evaluation of plant invader impacts are needed to identify problematic species before they become too widespread to effectively manage. While observational data can be readily gathered to identify negative relationships between invading and native species, these patterns are not necessarily indicative of invader impact and may instead reflect the legacy of past conditions influencing invasibility. We augmented standard observational methods by using local‐scale time‐since‐invasion information in conjunction with rapidly gathered plant abundance data to evaluate potential impacts of common buckthorn (Ramnus cathartica), a woody exotic in the early stages of invasion in Montana, USA. This species occurred as scattered populations of limited distribution and was proposed for listing as a state noxious weed, but empirical information on ecological impacts in the region was lacking. We recorded cover of understory and overstory plants across gradients of buckthorn invasion at 12 riparian sites representing five river drainages throughout the state. Uninvaded plots were located proximal to invaded plots at each site. Time since invasion per plot was approximated by aging buckthorn plants via annual rings. We found strong negative correlations between cover of native plants and buckthorn. In addition, buckthorn overstory cover increased with time since invasion, while native overstory cover decreased with invasion time, consistent with an impact scenario wherein the progression of invasion and associated increases in invader abundance suppressed native taxa. Although environmental factors that simultaneously promoted the increase of invaders and the decline of natives over time could have produced patterns mimicking invader impact, such a mechanism would have been more likely to manifest at broad scales to affect both uninvaded and invaded plots at a site. Our approach of using local‐scale time‐since‐invasion data to examine temporal signatures strengthened inferences made from standard observational methods and provided key input to support the listing of an emerging invader as a noxious weed in Montana. These results suggest that rapid empirical assessments of plant communities that consider time since invasion could be used to more confidently evaluate invader impacts and better inform the listing process for noxious and other regulated species.