The relationship between seed weight, dispersal ability and seed “quality” was tested for the seeds of pale swallow-wort Vincetoxicum rossicum (Kleo.) Barb., an invasive alien vine. In a field release, small seeds traveled significantly farther than large seeds. In one of two greenhouse experiments, large seeds were significantly more likely to germinate than small seeds. Seed weight also had a significant effect on seedling size at the time of harvest, but only when the seeds were grown in competition with a mixture of grasses. Seeds containing more than one embryo produced greater total seedling weight in the absence of competition, but polyembryony was not advantageous when seedlings were grown with grasses. We argue that if long-distance dispersers are less likely to germinate or survive, measured seed dispersal curves could lead to overestimation of the rate of spread of invasive plants.