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The water chestnut: exotic, invasive and insidious

Start of a water chestnut (Trapa natans) colony in an aquatic plant community – Pointe-au-Sable, 2020
Credit: Space for Life (Pascale Maynard)
Start of a water chestnut (Trapa natans) colony in an aquatic plant community – Pointe-au-Sable, 2020
  • Start of a water chestnut (Trapa natans) colony in an aquatic plant community – Pointe-au-Sable, 2020
  • Two water chestnut (Trapa natans) rosettes well concealed among Brasenia plants – Ile Jones, 2020
  • Dense mat of water chestnut (Trapa natans) – Baie de Carillon, 2020
  • Water chestnut (Trapa natans) rosettes bearing nuts in the fall – Voyageur Provincial Park, Ontario, 2013
  • Water chestnut (Trapa natans) rosette with fruits (nuts) developing – Baie de Carillon, 2020
  • Nuts, submerged leaves and floating rosette of the water chestnut (Trapa natans) – Voyageur Provincial Park, Ontario, 2007
  • Nuts of the water chestnut (Trapa natans) – Voyageur Provincial Park, Ontario, 2013
The water chestnut: exotic, invasive and insidious

Those beautiful little rosettes of floating leaves seem harmless ‒ and yet! Like numerous invasive aquatic species, the water chestnut (Trapa natans) proliferates quickly, spreads easily and can colonize lakes and waterways in a flash. The exotic species, which takes hold insidiously, can in no time at all dominate indigenous vegetation to the detriment of natural habitats.

The intruder

A native of Eurasia, the water chestnut was introduced into the United States in the late 1870s as an ornamental plant. It first spread through the northeastern states. In 1998 it was identified on this side of the border in the Rivière du Sud, a tributary of the Richelieu River. Since then it has been found in Québec in different spots such as along the Ottawa River.

A distinctive look

There’s no mistaking the water chestnut if you come across it in a lake. The plant is easily recognizable thanks to a characteristic floating rosette that can reach more than 30 centimeters in diameter.

The small triangular leaves that make up the rosette have spongy bulges that serve as floats. In July, the annual plant produces little white flowers that will give birth to fruits called “nuts” or “chestnuts.” These fruits, which have nothing in common with the water chestnut used in Asian cooking, measure three to four centimeters and sport four very sharp spines that give them a threatening look.

A champion of dispersion

Propagation of this exotic aquatic plant is tremendously efficient.

Each plant resulting from a nut can produce up to 15 rosettes. And each of these can produce 15 to 20 nuts. In other words, by the end of the season, a single plant can potentially produce up to 300 nuts, which enable it to survive the winter.

Moreover, when one of the rosettes is separated from the parent plant, it continues its growth and in turn anchors itself in the sediments.

At the end of the season, the floating rosettes detach and go adrift, carrying the nuts with them wherever the wind and the current take them.

Fishermen and pleasure boaters involuntarily play an active part in this dispersal. The water chestnut’s long stems get tangled in boats’ motors, and the nuts get caught up in in ropes and fishing equipment.

The plant is a tough “adversary”! Its nuts can remain in the sediments for 12 years before germinating. So it’s easy to understand why the struggle against water chestnut is likely to take quite some time if an infestation isn’t quickly contained.

Devastating impacts

The water chestnut is far from being harmless. As it goes about forming extremely dense floating mats, it reduces plant and animal biodiversity and makes activities like boating, fishing and swimming practically impossible. The hard spiny nuts on the lake or river bottom or on the shore can injure swimmers.

Some tips to stop its spread.

  • Learn to recognize it.
  • Do not plant it in a water garden.
  • Avoid boating in areas where it can be found.
  • Inspect and always clean your boat and your equipment when leaving a waterway and before returning it to the water. Never throw the plant in the water.
  • If you observe any plants, pull them up. If the colony is important, alert the municipality or the RCM and enter your observation on the Sentinelle tool.
  • Take part in uprooting activities in your area.

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