As an annual plant, cheatgrass relies on seeds to start a new generation each year.  When seeds are dispersed in early summer, they are dormant--that is, they will not germinate in response to summer rain.  They become nondormant by autumn, and are poised to germinate very quickly after it rains.  If there is not enough rain to trigger full germination, the remaining ungerminated seeds can follow one of two pathways.

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 Bromus tectorum seed headsgreen cheatgrass seeds


      They may sprout in spring, or they may enter secondary dormancy, which prevents spring germination.  They lose dormancy during the summer and are ready to germinate in their second autumn.  The latter group are "carryover seeds," an insurance policy in case the first year seedlings don't survive. 

cheatgrass dormancy cycle