Water Chestnut Facts and Control Methods

By David Newell, North-Eastern Lake Ontario Steward


Water Chestnut (Trapa natans) is an aquatic invasive plant species to the U.S. It is currently found in the states of New York, New Hampshire, Massachusetts, Vermont, and Rode Island. Water Chestnut is native to Europe, Africa, and Asia; it was brought into the U.S. sometime in the 1800s as a water garden plant, and it eventually escaped, thus has become an invasive species in many of the states listed above. It spreads by producing thorny seeds (nutlets) that can stick themselves to watercraft, the fur of mammals, or the feathers of birds. Unlike most of invasive plant species water chestnut cannot reproduce by fragmentation of the plant, only by seeds. Water Chestnut looks very different from other aquatic plant species. Water Chestnut is a submerged plant with floating leaves that have toothed margins and are triangular in shape with a waxy coating on top and hairy underside of the leaf.

Water Chestnut negatively effects the ecosystem and water recreation by forming “dense floating mats and out competes native plant communities. Its decay can deplete oxygen levels, leading to fish being killed. Dense growths can interfere with swimming and entangle propellers, which hinders boating, fishing, and waterfowl hunting.” (Water Chestnut, Minnesota Sea Grant).  This is why Water Chestnut should not be transported to other ecosystems, and why management practices for the plant are crucial. Water Chestnut starts to fruit in July when a white flower appears on the surface of the water. The fruiting bodies creates the nutlets that have four thorny points to it and eventually drop to the bottom of the water way. The nutlets can be hazardous to people and animals due to the four thorny points. Water Chestnut can rapidly reproduce and spread due to its seeds. One single Water Chestnut can produce 25 seeds. This allows Water Chestnut to rapidly take over a water way.

The four control methods for Water Chestnut is hand pulling, mechanical harvesting, chemical, and biological. The hand pulling method works well with small population of Water chestnut, the roots are shallow this means the plant is easy to pull out of the water. However; Water Chestnut is an annual plant species thus it is critical that you try and get the nutlets out of the water when you pull the plant up. The hand pulling method eliminates accidentally pulling native plants. They are normally attached to the plant but can fall off easily. Here’s Cayuga County’s guidelines for hand pulling Water Chestnut “Pull before seeds mature in mid-August, pull as much of the plant as possible, start at the edge of the infestation and work your way in. dispose of plant by composting on land or in the trash, coordinate hand pulling with mechanical harvesting, especially where large infestations exist, protect your toes” (Water Chestnut Control, W2O!). This method is very labor intensive. The next method is mechanical harvesting which works well for large areas of infestations, mechanical harvesting works by cutting and removing the plant from the water way. In order to be effective the harvester must harvest before the seeds drop into the water. A downside to the harvester is it can also harvest native plants along with the invasive ones, and possible injure or kill the aquatic life. This method is still very labor intensive and expensive, also leaves large amount of disposed plant material on land.

The chemical treatment is the use of herbicides for large infestation of Water Chestnut, the herbicides must be applied before the seeds drop, the downside to this method is it can have a negative impact of native aquatic life, and the treatment only works for current year’s growth, the herbicide cannot penetrate the seeds shell. This means that each year you need to come back with an herbicide and treat the Water Chestnut until the seed bank is diminished. “In New York State, chemical applications require a permit and must be done by a certified pesticide applicator” (Water Chestnut Control, W2O!). The last method is biological control; biological control requires that the invasive species is reunited with its predator species. This requires research to be done to determine if the predator species will only target the invasive species and not the native species that are beneficial to the environment. There is currently research being done by Cornell University on the biological control for Water Chestnut.

What can you do to help the control the spread of water chestnut?

Clean, Drain, and Dry your watercraft and gear.


Works Citied:

“Water Chestnut.” Minnesota Sea Grant. Sea Grant, 4 May 2016. Web. 5 June 2016. <http://www.seagrant.umn.edu/ais/waterchestnut&gt;.

“Water Chestnut Control.” W2O! Cayuga County, n.d. Web. 5 June 2016. <http://www.cayugacounty.us/portals/0/wqma/weedswatchout/controlwc.htm&gt;.


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