Phenology of two-horned water chestnut (Trapa bispinosa Roxb. var. iinumai Nakano)in northern Virginia ponds

Species of water chestnut, specifically Eurasian water chestnut (Trapa natans), have plagued the northeastern United States, including the tidal Potomac, for over 100 years. In 2014 a new species of invasive water chestnut identified as two-horned water chestnut (Trapa bispinosa Roxb. var. iinumai Nakano) was discovered in the Potomac River, and in subsequent years it has spread to nearby waterbodies. The purpose of this study is to describe the phenology of T. bispinosa to assist managers in developing effective approaches for management. Structured observa-tional studies were conducted at two ponds in northern Virginia in 2019 and 2020. Trapa bispinosa initiated growth in late April, increasing rapidly to a maximum of 100% cover in June. Rosette diameters increased gradually from late April to a maximum in August and September. This increase in rosette size was strongly correlated with degree days and calendar days and is consistent among ponds and between years. Flower counts were zero from April through June, then increased rapidly to maximum in late August. Fruit counts were zero from April through June; fruit started to appear in July, and counts increased to a maximum in early September. Since the species is annual and dependent on sexual reproduction, control efforts for T. bispinosa should be initiated before fruits are produced. Based upon our data, in the mid-Atlantic region, May would be an ideal time to begin because rosettes should be observable, but flowers and fruit should not appear until late June. These studies indicate aquatic managers may have a 4-to-6-wk window in the late spring to prevent seed production and should focus resources on management during that period.