I tested for the presence of an Allee effect in pale swallow-wort Vincetoxicum rossicum, an invasive alien vine, by planting 1-, 9- and 81-plant patches in an old field. Biomass and seed set of central "target" plants were significantly higher in the 81-plant patches than in the 1- and 9-plant patches. Biomass of roots did not differ significantly among treatments. The nine central plants of the 81-plant patches also had higher biomass and produced more seeds than the nine plants of the 9-plant treatment. Unlike most other examples of Allee effects in plants, the effect in V. rossicum was not due to differences in pollinator visitation rates. Instead, V. rossicum appears to suppress background vegetation more effectively when growing in larger patches. Like many other invasives, V. rossicum has been present in North America for years before it attained pest status. The Allee effect is one of several mechanisms that may contribute to such a latency period.