To assess the risk of gene movement between winter wheat and jointed goatgrass, information about the reproductive development of jointed goatgrass, winter wheat, and related hybrid plants is required. Seedlings from five jointed goatgrass populations, winter wheat, spring wheat, and jointed goatgrass by wheat reciprocal hybrid plants were exposed to 4, 7, or 10 C temperatures for 0, 2, 4, 5, 6, 6.5, 7, or 8 wk. Vernalized seedlings were transferred to a greenhouse set to 30/18 C day/night temperatures and 16-h photoperiod. Growth stages on all plants were recorded twice a week. All spring wheat and spring wheat related hybrid plants reproduced (as measured by the first reproductive node) in the absence of vernalization. Plants of jointed goatgrass population A-R, winter wheat, and winter wheat related hybrids were unlikely to reproduce in the absence of vernalization. Plants of jointed goatgrass populations B-W, G-S, E-S, and F-W reproduced in the absence of vernalization, and the likelihood that these plants would reproduce was different from all other plants. Plants that entered their reproductive phases together were not in synchronous development at anthesis. Plants in these studies differentially passed through the reproductive phases between the first reproductive node and anthesis. Our results demonstrate that variation in vernalization response exists among several jointed goatgrass populations, and reveal that the reproductive behavior of vernalized jointed goatgrass plants at anthesis is delayed compared to vernalized winter wheat and related hybrid plants. Hybrid plants produced between spring wheat and jointed goatgrass were vernalization insensitive. We hypothesize that hybridization between wheat and jointed goatgrass occurs as a result of cross-pollination between the younger reproductive tillers of jointed goatgrass and older reproductive tillers of wheat. The use of an early maturing wheat cultivar may exploit the difference in reproductive development and reduce the risk of hybrid production.