Invasive exotic plant | An uneven fight against water chestnut

It grows as bad grass and spreads on several lakes and rivers in southern Quebec. Come to the end seems almost an impossible mission, but several organizations of the watershed nevertheless lead a fierce battle against the water chestnut, an invasive exotic plant renowned among the toughest.

“It grows so fast!” No matter how much we tear it off, it grows even faster than all our efforts to make it disappear, ”says Marlène Herzog, an environment technician at the Council of Basse Les Mille-Îles, the Cobamil.

“That” is the water chestnut, an invasive exotic plant, which is aptly named in Carillon bay, at Lac des Deux Mountains, west of Montreal. The plant forms a huge green carpet in the bay, which can even be seen on satellite images.

This plant is particularly harmful to biodiversity. It occupies the space so quickly that it does not leave room for native plants in a stream. It reduces the light necessary for the growth of other plants and reduces the level of oxygen in water. Its presence can prevent swimming or fishing. It appears on the list of pervasive floristic exotic species deemed priority in Quebec.

With her colleague David McConnell, Marlène Herzog coordinates a team of ten people this summer, which leads an intense campaign to tear the water chestnut.

But like Lerne’s hydra, this snake with several heads of Greek mythology, the water chestnut is a formidable adversary, notes Mr. McConnell. “No matter what efforts we put, it only makes you fat,” he said, making the same observation as Hercules, the son of Zeus, who saw two snake heads pushing after having cut one with his sword.

Since 2020, the different Cobamil teams have still waged 182 tonnes of plants in Carillon bay, the equivalent of a very large blue whale. These data do not include the 2025 assessment which will be carried out at the end of the summer.

The uprooting campaign continues nevertheless even if the efforts made sometimes seem in vain. “There are areas where there were no water chestnuts and there are now,” says Marlène Herzog. “You usually have to go back to the same place several times to tear plants to hope to reach a result,” adds David McConnell.

A green carpet

But how can a plant give so much a hard time? A single plant of Trapa swimmingthe scientific name of the water chestnut can produce up to 15 rosettes and each rosette can give between 10 and 20 nuts, which can survive up to 10 years at the bottom of the water. “A single nut is 200 to 300 new plants that will appear,” says David McConnell.

In 2023, the green carpet formed by Trapa swimming occupied an area of 35 hectares, the equivalent of 50 soccer courts. But since then, it has progressed and occupies an even larger area, specifies Marlène Herzog. The Cobamil estimates that there are tens of millions of rosettes of water chestnut in Carillon bay.

The plant also benefits from climate change, which create conditions increasingly favorable to its growth thanks to warmer waters.

If the task seems almost impossible, it is mainly because manual uprooting is far from being sufficient with such a prolific species. In the summer of 2021, the efforts made it possible to withdraw 17 tonnes of plants, but the balance sheet improved significantly in 2022 with the use of a faulty summer, a machine specially designed to remove aquatic plants.

At the end of the summer of 2022, 119 tonnes of water chestnuts were torn off, an assessment that has not been equaled since. “We started with the Faucarduse in 2021, but 2022 was the only year when we were able to use it throughout the summer without having mechanical troubles,” notes Marlène Herzog.

When visiting The pressthe Faucarder could not be used due to a mechanical break. The device should probably be replaced, David McConnell believes.

The aircraft was bought for $ 20,000 and nearly $ 70,000 was invested in repairs, said Cobamil Director, Raphaël Goulet.

Handworming

That day, therefore, the team waged the plants by hand. Dressed in fishermen’s pants, Cobamil employees advance slowly through a carpet of water chestnuts, which they remove one by one before placing them in a canoe.

The plants will then be transferred to a container, which will take the path of the landfill site once full. There is no question of sending it to the composting: it is feared that seeds can go to contaminate other rivers.

In areas where the water is too deep to walk with fishermen’s pants, failing to use the faulty, the only option is to use a fork or a rake to tear the water chestnut.

While we are on the water aboard a pontoon transformed into a barge, David McConnell goes there with a demonstration. He uses his rake to hang several plants and then pulls to bring them back to the deck. The root of a single plant can easily be 5 meters long. “We get tired quickly with this technique,” says David.

The Cobamil benefits from funding from the Quebec Ministry of the Environment until 2030, but Marlène Herzog and David McConnell do not see how they can overcome this plant without reinforcements. “We would need two full -time falcarders to eradicate it,” says Marlène Herzog.