Downy brome (Bromus tectorum L.) is a widely distributed invasive winter annual grass across western North America. Bromus tectorum phenology can vary considerably among populations, and those differences are considered adaptively significant. A consensus hypothesis in the literature attributes the majority of observed differences in B. tectorum phenology to differing vernalization requirements among populations. A series of greenhouse experiments were conducted to identify differences in B. tectorum vernalization requirements and link vernalization to expression of annual false-brome [Brachypodium distachyon (L.) P. Beauv.]-derived vernalization gene homolog (BdVRN1). Results from this study indicate that variation in time to flowering is partially governed by differing vernalization requirements and that flowering is linked to the expression of BdVRN1.