The timing of seed fall and the innate germinability of newly shed seeds may jointly or separately determine whether they germinate immediately upon landing on a suitable substrate or rather are dormant until a subsequent growing season. In 2 years studies, most seed fall of Lythrum salicaria occurred during a 6-8 week period beginning about early November. The seeds were released when daily average temperatures were generally <15 o C, the reported threshold temperature for germination. Seeds sampled during autumn and winter exhibited enhancement of germination by storage under cold moist conditions, and this effect was most pronounced for seeds gathered early in the seed dispersal period. Seeds gathered late in the season had a fairly high immediate germinability, apparently through enhancement by winter weather. Since either dissemination phenology or innate dormancy would prevent premature seed germination in this population, the prolonged retention of loosestrife seeds on the maternal plant most likely serves primarily to effect temporally staggered dispersal.