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Photo By Thom Smith Water Chesnut has a rosette of floating, fan-shaped, triangular leaves, which from a distance somewhat resembles a bright water lily leaf.

Q: What invasive noxious aquatic weed is now found in New England in Connecticut, Massachusetts, Vermont and New Hampshire (as well as neighboring New York State, and has also gotten a stronghold in Pennsylvania, New Jersey, and well north into eastern Canada (mostly Quebec)?

A: Trapa natans, the Water Chestnut, and if a picture is worth a thousand words, I wouldn't hazard a guess as to how many more, two videos would be. For readers with Internet access, enter this address in your browser and hopefully it will result in a web page with two videos on this invasive species:

Trapa goes by various names, European water chestnut, water nut, horned water chestnut, water caltrop, and bull nut, but regardless of what you call it, once it gets into a slow moving, shallow of water (pond, lake, river, oxbow) it is nearly impossible to entirely remove. Even in lakes like Pontoosuc (Town Brook) in Lanesborough, Mass., efforts to remove it for a number of years will still need watching and remediation for years to come. Seeds can last for more than 10 years buried in sediment. And it may also be re-introduced by geese or watercraft. Geese have been seen with seed bearing fruit, a large nut with four sharp spines attached to feathers.


Water chestnut prefers nutrient rich, slow moving shallow waters, although it may have branching stems reaching as much as 16 feet. Its most obvious feature is a rosette of floating, fan-shaped, triangular leaves, that from a distance somewhat resembles a bright water lily leaf, but upon closer inspection reveals its rosette of triangular leaves both on the surface and submerged. The surface leaves have long stem-like petioles with inflated air bladders that keep the plant afloat. Flowering begins in mid to late July with "nuts" called caltrops ripening about a month later that detach and sink to the bottom. If conditions are correct they may float to another area. Flowering and seed production may continue well into the autumn, until frost kills the floating plants. It can be pulled up if done so gently. Anyone doing this must bag or compost the plant(s) so they do not have an opportunity to return to water. It is best to contact local fisheries officials with location rather than taking on removal.

When I was actively searching out this species (via kayak) for an article in Mass Wildlife Magazine a number of years ago, I saw early invasions at Three Mile Pond in Sheffield, Mass., the middle basin at Hoosac Lake in Cheshire, Mass., and well established at Pontoosuc Lake in Lanesborough and Onota Lake in Pittsfield, Mass., among others.

Water chestnut was first introduced to North America at the botanical garden at Harvard University in 1877. The plant escaped cultivation and was (unofficially) distributed. It was found growing in the Charles River by 1879. The plant was then introduced into Collins Lake near Scotia, NY (in the Hudson River-Mohawk River drainage basin) around 1884, probably intentionally, among other places.

This species is not the same as the "water chestnut," which can be purchased at the supermarket and is used in Asian cooking.

Thom Smith welcomes your questions and comments. Email him at or write him care of The Berkshire Eagle, 75 South Church St., Pittsfield, MA 01201