We continue our summer Aquatic Invasive Species “Creature Features” with the European Water Chestnut. Alongside the invasive aquatic plant species known as phragmites (see our Phragmites Creature Feature Article), increasing incidences are being reported in Eastern Ontario of a similarly uninvited and destructive water-borne plant, known as the European Water Chestnut (Trapa natans).
The Ottawa area has been particularly hard hit, but since most of the water systems in eastern Ontario are connected, it’s only a matter of time before this species becomes more prevalent in the Cataraqui Region as examples of the plant have been found on Wolfe Island, and in the Cataraqui River south of Kingston Mills.
Why is this plant a problem?
According to a report on the Ontario Ministry of Natural Resources & Forestry website, the European Water Chestnut “forms extremely dense floating mats of vegetation that shade out native vegetation, decreasing plant biodiversity and making recreational activities, like swimming, angling and boating almost impossible in the infested areas.”
As well, “the hard nuts with barbed spines can accumulate on shore and cause injury when stepped on,” and “reduced light penetration and plant growth beneath the water chestnut canopy, combined with a large amount of decomposing vegetation below, can lead to decreased dissolved oxygen levels, which can impact native species and cause fish kills.”
To help curb the spread of this invasive plant, its important to identify it, especially before it becomes dominant in a particular water body.
One of the best descriptions comes from www.invasivespeciescentre.ca:
“European water chestnut is an aquatic plant with a hard stem that can reach up to 3-5 m in height. The leaves sit above the water, spread out in a circular pattern, and are triangular with serrated edges. In June, European water chestnut blooms small white flowers with four petals. Water chestnut can thrive in lakes, rivers, streams, and ponds with soft substrate, full sun, and nutrient-rich waters. It is most abundant in water that is approximately 2 m deep but can be found in water up to 4 m deep.”
And to be clear, this is not the same sort of edible water chestnut that you can buy at grocery stores (and which are particularly delicious roasted in the oven wrapped in bacon!) The Trapa natans variety is native to Western Europe, parts of Africa and northeast Asia (parts of Russia, China and down as far south as Indonesia).
Research, as reported in various scientific and government websites (including that of the Ontario Ministry of Natural Resources, the Ontario Invasive Plant Council, Ontario Federation of Anglers and Hunters and the Ontario’s Invasive Species Awareness Program), there is evidence that the European Water Chestnut was introduced to North America in the late 1870s, when it was planted in a botanical garden on the grounds of Harvard University in Cambridge, Massachusetts, in decorative ponds in neighbouring communities, as well as parts of New York State. By 1900 it had become well established in the Hudson River valley and was reported on the American side of Lake Ontario and Lake Erie in the 1950s.
So how can the negative impacts of European Water Chestnut be mitigated in the Cataraqui Region and beyond?
Obviously, removing as much of the plant as possible from water bodies would go a long way, especially if the removal happens to newly sprouted patches.
The Ontario Invasive Species Council says that “early detection of introductions and a rapid control strategy are key to preventing high-impact infestations. Because T. natans is an annual plant, effective control can be achieved if seed formation is prevented.”
New York State Department of Environmental Conservation is one of many governmental bodies on both sides of the U.S./Canada border outlining the best practices by which European Water Chestnut can be removed manually or mechanically.
Hand-pulling/Manual Removal –
- Hand-pull rosettes when they first appear (mid-June to early July), before seeds are developed.
- Remove as much of each plant as possible, including the stems, any developing seeds, and roots.
- Avoid pulling native aquatic plant species, if possible.
- Identify access areas, disposal site, and location for storing harvested plants during the drying process.
- Start at the edge of the infestation and work towards the center.
- If possible, revisit the site repeatedly to pull plants that continue to sprout throughout the growing season.
- Repeat plant removal early each growing season until the seed bank has been depleted.
- Monitor site for a minimum of 12 years to ensure seed bank has been depleted and no new introductions have occurred from other sources.
Mechanical Removal/Machine Harvesting
- Conduct an aquatic plant survey to determine whether harvesting would also eliminate native and/or protected species.
- Identify access areas, disposal site, and location for storing harvested plants during the drying process.
- Harvesting machines can be used when rosettes first appear (mid-June to early July) before seeds are developed.
- Remove as much of each plant as possible, including the stems, any developing seeds, and roots.
- Repeat plant removal early each growing season until the seed bank has been depleted.
- Monitor site for a minimum of 12 years to ensure seed bank has been depleted and no new introductions have occurred from other sources.
- Avoid mechanical harvesting if water chestnut is accompanied by other invasive species that reproduce via fragmentation such as Eurasian watermilfoil.
Other things you can do to prevent the spread of invasive aquatic species include never buying or keeping European Water Chestnut in your water garden or any aquarium. It’s not simply good advice, it’s also the law – as it is illegal to buy, sell, trade, possess or transport this plant.
For boat owners/uses, it’s important to clean, drain and dry the boat, trailer and accompanying equipment after each use. If any European Water Chestnut is found during this process, dispose of the plants in refuse containers that are away from the water body before transporting the boat over land.
Invasive aquatic plant and animal species, including many types of fish, have been introduced by people dumping unwanted aquarium water (plants, fish and all!!) into local lakes, rivers and streams. So, please don’t do this.
Cataraqui Conservation staff say the best thing to do if you find examples of European Water Chestnut or indeed any other invasive plant or animal species in the wild, is to contact the Invading Species hotline at 1-800-563-7771, visit EDDMapS, or search for the Invasive Species in Ontario project on iNaturalist.org and make a report.
For more information on the situation in the Cataraqui watershed, please contact info@crca.ca for more information.