Lythrum salicaria L. (purple loosestrife ) is a rapidly spreading, naturalized perennial herb of moist habitats and is considered detrimental to wetland productivity in North America. The first herbarium record for Manitoba was collected from Neepawa in 1896. As of 1991, 38 sites in Manitoba from field work, correspondence and herbarium records were identified. Most of these were in the watersheds of the Red and Assiniboine Rivers. Dispersal is mostly by means of seed. Significantly more seed matures in the capsules in the lower and mid regions of the infructescence compared to those from the top. Seed is shaken by wind from the capsules between late summer and the end of winter. Germinability of seed from 28 cultivated and naturalized populations averaged 92 percent, however cultivars had low seed production (especially "Morden Pink"). Discriminant analyses identified calyx pubescence calyx lobe length, and calyx appendage length as significantly reliable features for separation of cultivars from naturalized plants. The cultivars "Morden Pink", "Dropmore Purple", and "Morden Gleam" were self-incompatible, but artificial crosses between these and a naturalized population when legitimate were for the most part fertile. Resulting seeds averaged 98 percent germination and the hybrid plants were highly interfertile. Thirty-seven percent of the hybrid progeny were classified in discriminant analysis as being indistinguishable from a naturalized population which grows on the Assiniboine River. Controlling the spread of L. salicaria will be accomplished by preventing seed production in naturalized populations, and by removing cultivars from locations where they may affect pollen transfer with naturalized populations.