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One great obstacle to understanding the invasion of nonnative species into native ecosystems is the lack of information on the population biology of the invading species. In particular, morphological and physiological adaptations and potential for phenotypic plasticity will strongly influence a species’ ability to persist and spread in newly invaded ecosystems. Phenotypic plasticity can buffer populations from selection thereby allowing them to survive the establishment phase of the invasion. The annual grass Aegilops triuncialis (Poaceae, Triticeae) has become highly invasive in California (USA) and provides an opportunity to investigate the importance of phenotypic plasticity to persistence and spread in new habitats. This species produces dispersal units containing dimorphic caryopses (‘‘seeds’’) with different degrees of dormancy. Germination of the smaller seeds is suppressed by maternal tissues and by the larger sibling, but how this induced dormancy varies across populations is unknown. I used 12 populations from northern California to compare size relationships between seeds and to investigate variation in the strength of the maternal and sibling effects on germination. The larger seeds were never dormant while induced dormancy of the smaller seeds varied across populations and years. Thus, the factors governing induced dormancy were not genetically fixed, but appeared to be environmentally influenced. For A. triuncialis, reproductive strategies, such as seed-size dimorphism, may facilitate initial invasions into new environments, but variation in the strength of controls over germination may be important for facilitating post-invasion persistence and spread.