Plant invader genotype determines much about how the plant establishes, spreads, and impacts receiving habitat and the types of management required to achieve its control. We investigated phenology, vegetative spread, growth and biomass allocation, and response to herbicides of four flowering rush (Butomus umbellatus L.) amplified fragment length polymorphism (AFLP) genotypes (triploid: GT1; diploid: GT3, GT4, GT5) in a multiyear mesocosm experiment. Monthly observations were made over 31 mo to determine whether there were differences in timing of peak plant growth and flowering. We also applied herbicides during years 2 and 3 using imazamox or triclopyr at rates previously determined to be efficacious against B. umbellatus. By the end of year 1, all genotypes spread vegetatively within their containers. From a single sprouted axillary bud (i.e., bulbil), GT4 plants spread to cover nearly 2,000 cm2 in 6 mo. Flowering occurred May through October for all genotypes, but was reduced in years 2 and 3, including almost no flowering in triploid plants after year 1. Maximum leaf abundance occurred in August or September for all genotypes. After one season of growth, biomass was greatest for diploid genotypes, allocated primarily to leaves. However, by year 3, triploid plant biomass was greater than diploid GT4 and GT5, but not GT3. Bulbil production occurred only in diploid genotypes. At the study conclusion, GT4 and GT5 produced nearly 15,000 and 20,000 bulbils each, whereas GT3 plants produced fewer than 5,000 bulbils. Herbicides were effective in reducing biomass, but triploid plants were far more susceptible than diploid plants. Propagule production was generally reduced by herbicides, but rhizome bud production increased 240% in triploid plants treated with imazamox. These results demonstrate (1) the significant capacity for reproduction and spread of B. umbellatus and (2) the importance of investigating differences between lineages of B. umbellatus to inform management and risk assessments.