The water chestnut is a rooted, floating aquatic plant. It is an annual plant not native to the United States, categorized as an invasive species in the Connecticut River Watershed. It is fast growing and quickly reproducing. If left unattended it will easily cover an entire waterbody. 

If it is not native, how did it get here?

European water chestnut has not always lived in the United States. The water chestnut is a resident of the Old World, native to Northeast Asia, Southeast Asia, Western Europe, and Western Africa. In 1877, the species Trapa natans was introduced to the United States from Europe (as suggested by its name) at the Cambridge Botanical Garden at Harvard University (Sculthorpe, 1967; Oliver, 1871; Voroshilov, 1982). It was planted in Collins Lake, MA and other ponds in Massachusetts. In 1879 the plant escaped the cultivated areas and started growing in the Charles River. Since then it has continued to spread all along the east coast as far south as Virginia and Kentucky, as far north as Quebec, invading the Hudson River and, of course, the Connecticut River. Without removal it has the capacity to spread farther. 

What does water chestnut look like?

Water chestnut plants its root at the bottom of the river, while their leaves float on the water surface. It has rosettes of leaves that float on the surface of the water that appear to be radiating from a central point. The leaves are triangular or slightly diamond shaped, toothed on two sides and connected to the stem by a long, flexible submerged stalk. Submerged leaves are feathery. The stem can grow up to 4.6m long, allowing them to colonize a wide-range of freshwater habitats — from shallows to deep waters. Plants typically bloom in July. The tiny, white, four-petaled flowers produce characteristic horned seeds. 

In lightly infested areas you might find a single rosette or plant floating on the water. In heavily infested areas water chestnut forms dense floating mats, often as thick as three layers. 

How does water chestnut reproduce?

Each plant produces at least one flower annually. These flowers begin to form nut-like fruits in mid-July. Seeds ripen in about a month and start to drop around early-mid August. The seeds, known as water caltrop, are four-horned nut-like structures that develop on the underside of the floating rosette. One seed can produce up to 15 floating rosettes, each rosette producing up to 20 more seeds. Seeds remain viable for up to 12 years. 

It is important to remove water chestnuts before seeds mature and drop to the floor of the water body. If we can prevent plants from dropping new seeds, an infestation can be successfully eliminated. 

Can water chestnut spread?

Seeds can drop directly into the sediment beneath the parent plant or they can hitch rides to new locations. Water chestnuts’ spikes allow them to stick to birds or other wildlife and end up in a new location. 

Plants can also get caught on boats or other recreation gear allowing them to get carried up/down-stream or to new water bodies.

Why is water chestnut such a problem?

To aquatic plants and animals: Water chestnut is a problem because it hogs space and nutrients. It can crowd out native plants that are food sources for native animals. Not only do they take over our waterways by out-competing other emergent and floating vegetation, their colonization negatively impacts the function of the entire aquatic ecosystem. They proliferate profusely on the surface of freshwater. The densely-layered floating mats they form — often as thick as three layers — limit light penetration through the vertical water column. As water chestnut decomposes, it decreases dissolved oxygen that fish and other aquatic life need to survive. 

To humans: Water chestnut infestations also impede fishing, hunting, swimming, and boating as the rosettes cover entire bodies of water with plants up to sixteen feet deep in some extreme cases. They congest streams, block boats, and kill fish. The spiny seeds can cause injury.  Management of out of control infestations is costly and requires incredible coordination. 

How is water chestnut managed?

Fortunately, it is easily identifiable and smaller infestations can be managed with trained volunteers hand-pulling the plant. The key to keeping water chestnut from invading new areas is to remove plants before they have a chance to set seeds. 

Control requires vigilant patrolling and harvesting for many years to ensure a water body is saved. Infested sites should be monitored and controlled every few years to fully remove the invasive species.

Most removal is completed by volunteers manually through a series of pulls. Volunteers pull the plants by hand, often depositing the pulled plants into baskets or boats that are brought to shore. Water chestnut can be composted away from the water body.  Mechanical harvesters are efficient for removing large infestations of water chestnut. They can remove lots of plants from the surface. However, this method can be quite expensive. It cost the state of Vermont approximately $500,000 to remove water chestnut in 2000. Compared to hand removal, which usually involves many volunteers and few expenses, this method is more expensive but less time consuming.

Who manages the removal?

Volunteers that frequently scout for and remove the invasive plants are the major fighters preventing heavy infestations. CRC, partners, and other stewards of our rivers collaborate to organize many water chestnut removal events. These community events have been very successful at removing large swaths of water chestnut. In the summer of 2019, volunteers removed over 4,762lbs of water chestnuts! 

You can learn how to identify and report water chestnut as well as when it is appropriate to pull the plants, how to properly pull them, and how to properly dispose of the pulled plants. Sign up as a volunteer or we can come talk to your organization! Contact us at crc@ctriver.org to schedule a presentation. Check out this webinar (2018) for more information about water chestnut, where it is in the Connecticut River basin, and how to help control it.

Management Organizations and Individuals