Utricularia purpurea (Purple Bladderwort) has had sporadic population explosions in some northern Wisconsin lakes within the last 20 years. It can form dense floating, nonflowering masses that impede recreational activities on the affected lakes. Three lakes in which this weediness was noted are soft water, low-nutrient, seepage lakes that experience widely fluctuating water levels and recent nutrient runoff. Excessive weediness has not been described in nearby Minnesota nor Michigan, although the plant is sporadically locally abundant. U. purpurea is listed as 'Endangered' in Minnesota.