Phenology of Two-Horned Water Chestnut Following Three Management Strategies: Herbicides, Harvesting, and Water Level Drawdown

Two-horned water chestnut, also known as Trapa bispinosa Roxb. var. iinumai Nakano, is an invasive freshwater macrophyte recently introduced to the Washington, DC Metro region. A collection of governing bodies is working to control the plant, each using a different methodology. The purpose of this observational study was to evaluate the effectiveness of each management technique, including manual harvesting, herbicide application, and water level drawdown. Observations were made during the growing season to measure the density and fecundity of the plant in response to treatment, as well as water quality metrics. Recovered coverage and reproductive bodies were recorded at 2 sites treated with manual harvesting and at 3 sites treated with the herbicide flumioxazin, with only a delay in phenology; these treatments will require repeat applications to be effective within the season. 0% cover and 0 mature fruits were observed at 1 site treated with water level drawdown, 1 site treated with the herbicide fluridone, and 2 sites treated with the integrated use of fluridone and flumioxazin. These results indicate that the most effective management technique will be determined by the size of the water body and the extent of the Trapa colony, and treatment must be maintained through the growing season to prevent seed production.