Wednesday, July 23, 2014
Water Chestnut (Trapa natans) Control

Water chestnut is an annual plant. The only part of the plant that survives the winter is the seed. This means if you can limit the number of seeds that are produced by the plant, you can limit the amount of water chestnut plants in future years. This can be done by removing the plants before they have a chance to set seed. Because seeds can remain in the sediment for several years before sprouting, it may take more than one season of control efforts before the "seed bank" (seeds that have accumulated in the bottom sediment) becomes depleted. Fortunately water chestnut does not reproduce by plant fragments.

Water Chestnut Nutlet, Kelly Fallone, Cayuga County CCE
Water chestnut nutlet, Kelly Fallone, Cayuga County CCE

Currently, water chestnut can be controlled using mechanical/physical and/or chemical controls, depending on the water body. No matter which method or combination of methods are chosen, control must occur before seeds drop from the plant. Research is currently underway to find a suitable biological control agent for water chestnut.

Mechanical/Physical Control of Water Chestnut

Hand Pulling

Hand pulling water chestnuts is not like pulling "baked in the ground" weeds from a garden. The roots of water chestnut are very shallow, so pulling is easy and very satisfying. Hand pulling only targets undesirable plants so it can be safely used in any environment. However it is labor intensive and may not be practical for large, established infestations.

Guidelines for hand pulling:

  • 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! Wear old sneakers.

    Hand Pulling Water Chestnut in Sterling Creek
    Hand Pulling Water Chestnut in Sterling Creek

    Hand Pulling Water Chestnut in Sterling Creek
    Hand Pulling Water Chestnut in Sterling Creek

Harvesting

Large areas of water chestnut may be better managed with a mechanical harvester. Harvesting water chestnut with a weed harvester is comparable to "mowing a yard". Harvesting water opens waterways for use by cutting and removing plants from the water. To be effective, the plants must be harvested before seeds drop. However, weed harvesters cut native/beneficial plants as well as invasives and can kill fish and other aquatic life. Weed harvesters are expensive and labor intensive to operate and large amounts of plant material must be disposed of on-land.

Chemical Control of Water Chestnut

Herbicides are another tool available to manage water chestnut. Herbicides may be more practical than hand-pulling for large infestations but their use is limited by waterbody characteristics. Also, herbicides may negatively impact non-targeted plants and animals.

Like hand-pulling and mechanical harvesting, herbicides are used to eliminate the current year's growth before seeds are produced. Herbicides are not able to penetrate the hard shell of water chestnut seeds, so treatment will be required until the seeds accumulated in the sediment have sprouted.

What rules apply? In New York State, chemical applications require a permit and must be done by a certified pesticide applicator. Monitoring and signage is also required. The only herbicide approved for treatment of water chestnut is 2-4D.

  • For permits on chemical control and additional information, contact the NYS Department of Environmental Conservation Bureau of Pesticides: 1-800-388-8244. The permit process takes about 6 months, so plan accordingly.

  • 2-4D is a systemic, selective herbicide which means it is absorbed by the plant and distributed throughout the roots, stem and leaves of only certain species of plants. 2-4D, selectively kills broad-leaved plants. Up to 75% of the water chestnut in a treated area will brown, wilt, shrivel and die. Any survivors can be pulled. Monitoring is typically required to make sure there are no unintended side effects.


Biological Control of Water Chestnut

One reason some non-native plants get out of control is because they have no predators in their new environment. The purpose of biological control is to control the problem species by reuniting it with its predator. Any time non-native biological herbivores are introduced to an area in efforts to control an invasive plant, much research must be done in advance to ensure that the herbivore prefers the plant "exclusively" to avoid herbivory of beneficial, native plants and future invasive species issues. Research is currently under way at Cornell University to find a biological control agent for water chestnut.

How Can I Help?

  • Learn to I.D. the water chestnut plant.

  • Remove plants from your boats and your trailers.

  • Drain bilge water, bait buckets, etc.

  • Pull it and dispose of it on land away from the shore or in the trash.

  • Participate in a hand pulling day.

  • Become a weed watch volunteer.

For more information, see Water Chestnut Alert (PDF).