Water Chestnuts in Northern Virginia


Picture courtesy of Blythe Merritt

William Rantis

On November 15, a meeting was held at Twin Lakes Golf Course concerning the alarming growth of an invasive species in Virginia. This meeting was convened to deal with a specific problem: the growth of water chestnuts in Northern Virginia. This invasive species is rapidly making headway, and this meeting was to determine some ways to stop its spread.

There have been two kinds of water chestnuts in Virginia now. Trapa natans was the first to make it to the Northern Virginia areas, making its first major appearance in the Potomac River in the 1920s. Its initial appearance was found to be negative for the rest of the river community. It grows from the bottom of the river bed and spreads out over the surface of the water in shallow areas. Many of them doing this obstructs light from reaching lower submerged aquatic vegetation, which also exists in shallow waters.

Submerged aquatic vegetation (SAV) is an important part of the underwater ecosystem. SAVs provide their underwater habitats with shelter for smaller fish, prevent erosion, and oxygenate the water. Having native vegetation leads to a stable underwater community while a lack of them can put the rest of that ecosystem in jeopardy– reducing numbers of other species. Once they’re gone, it takes longer for them to recover as well. For example, after the Trapa natans was harvested and removed from the ecosystem by the Army Corps of Engineers and removed from the ecosystem in the 1950s, it took multiple decades for the SAVs to repopulate.

Now, a new type of water chestnut is making its way up Virginia waterways. Going by the name Trapa bispinosa, its population has consistently doubled year after year. In most ways, this new type of Trapa is pretty similar to the natans variant. Some notable physical differences include that it flowers pink (while natans flowers white),  as well as the fruit of Trapa bispinosa having two horns compared to Trapa natans’s four horns. The spread of this variant, if allowed to progress, could prove just as harmful as the former one.

The water chestnuts spread the seeds by two main methods: by flowing water and by animal carriers. Rivers carry the seeds downstream or the seeds get stuck to geese or other birds. Once they arrive at their destination, they sink to the bottom and will later grow into a new plant. If the seeds are not carried downstream or picked up by birds, they settle close to where they were released–making the area around the initial sprout more crowded. If they are picked up by birds, they can be brought to close by ponds in which they establish new colonies. Seeds may lie dormant for a few years before sprouting. If left to their own devices, water chestnuts can completely cover shallow ponds and greatly cover others.

There are a few ways of controlling the water chestnuts after they have established themselves in a body of water. One way is to send out people to manually pull them up, but this takes a large amount of time and effort. Using herbicides is an option, but that may come at the cost of damage to other plants in the ecosystem. The last method is to use machine harvesters to remove them. This requires the least amount of man-hours and minimal environmental damage (but the machines are not always available).

Water chestnuts are like fire, in that they are exponentially more difficult to remove and contain when they have spread. If they are spotted before they have dropped seeds, they are also easier to contain, as there will be no chestnut reemergence. In order to make sure that wildlife management is aware of new growths of water chestnuts, if you see water chestnuts in ponds, report such findings to John Odenkirk at john.odenkirk@dgif.virginia.gov.