Cereal rye (Secale cereale L.) is grown as a cover crop due to its ability to enhance soil health and suppress weeds through allelopathy, but germination responses of different weed species to allelochemicals have not been studied for seeds varying in age and consequently vigor. This study investigated the allelopathic effects of cereal rye on the germination of Palmer amaranth (Amaranthus palmeri S. Watson), large crabgrass [Digitaria sanguinalis (L.) Scop.], giant foxtail (Setaria faberi Herrm.), and lettuce (Lactuca sativa L.). Seeds were germinated in vitro in media with allelochemicals secreted by roots of cereal rye lines varying in allelopathic activity. Seeds were subjected to an accelerated aging treatment to modify their vigor. Results showed that aged seeds exhibited 31% lower germination than non-aged seeds. The magnitude of the germination reduction due to the presence of allelochemicals was species dependent. In the absence of allelochemicals, L. sativa exhibited a 20% reduction in germination due to aging, while the reduction was greater than 60% when allelopathy and aging were combined. Non-aged S. faberi seeds increased germination under low allelopathy conditions, with 20% greater germination compared with the non-treated control. Digitaria sanguinalis did not respond to seed aging, and the high allelopathy treatment reduced germination less than 10%. Amaranthus palmeri exhibited the highest germination and was the only species that was not affected by the seed aging and allelopathy treatments. Germination rate was reduced by seed aging and to a lesser extent by allelochemicals. Results suggest that incorporating allelopathic S. cereale varieties in cover crop rotations can reduce weed populations. However, the age structure of the seedbank might determine the importance of allelochemicals for emergence at the species level, likely causing weed community shifts.