The Detroit Lakes chain of lakes consists of five basins in northwest Minnesota adjacent to the town of Detroit Lakes. Flowering rush has been established in these basins since the 1960s. We evaluated the distribution of flowering rush in the five basins using a point intercept method, with 830 points distributed in a grid with points 150 m apart. These data were analyzed to determine whether invasive and native species frequencies were different between 2010 and 2011. We also assessed co-occurrence of flowering rush with native hardstem bulrush. The distribution of both flowering rush and hardstem bulrush was unchanged from 2010 to 2011. Flowering rush is invading areas with native plants and not establishing in unvegetated areas. Although flowering rush is found as deep as 4.5 m, it is most frequent at a depth of 1.3 m. We also examined the distribution of biomass and growth across a depth gradient from 0.3 to 3.0 m in 0.3-m intervals. At each 0.3-m interval, three biomass samples were collected at each of 10 transects for a total of 30 samples per depth interval or 300 biomass samples. At each point, leaf height, emergent leaf height, water depth, number of ramets, and number of rhizome buds were counted. Biomass samples were collected in a 0.018-m² core sampler, sorted to shoots and belowground biomass. We found that flowering rush height and biomass peaked at 1.3 m and declined with greater depth. Bud density was negatively related to water depth. Bud density averaged 300 buds m⁻², which was three times the average ramet density (100 ramets m⁻²).

Nomenclature: Flowering rush, Butomus umbellatus L.; hardstem bulrush, Schoenoplectus acutus (Muhl. ex Bigelow) Á. & D. Löve.

Management Implications: Flowering rush is an invasive aquatic plant that has established in water resources across the United States and Canada. Flowering rush has been established in the Detroit Lakes (MN) since the 1960s and become a significant nuisance to shoreline residents and recreationists. In this study, we document that, although flowering rush can occur to depths of 4.5 m, it was most abundant to 1.3 m, so management efforts could be targeted from the shoreline to depths of 1.3 m. Flowering rush established in areas that already contained native aquatic plants, including hardstem bulrush, rather than establishing in areas without vegetation. Examining biomass allocation in depths from 0.3 to 3.0 m, biomass of shoots and density of ramets was highest in depths less than 1.0 m and decreased significantly beyond 1.3 to 1.6 m. Rhizome buds, the main propagule of flowering rush, had their highest densities from 0.3 to 1.6 m, averaging 200 to 300 buds m⁻², which translates into 2 to 3 million buds ha⁻¹. Management can be focused on water depths of 1 m or less to target the densest infestations of flowering rush, which will serve to reduce the pool of propagules for further spread or reinfestation.