The plant is practically invincible and its growth, exponential. Japanese knotweed, an enemy of building owners and native flora, is silently colonizing Quebec. Getting rid of “that perfect plant” is hell for anyone who finds this pest in their backyard.
The grove exceeds Valérie Tchang by more than a meter. His team uproots the stems one by one, then digs up the roots before covering everything with a black tarp. “It’s very difficult and it’s discouraging,” she breathes.If we don’t do anything, in two weeks it will grow back two feet. »
The damage from this Japanese knotweed infestation on the edge of this east Montreal home is clearly visible. The branches pierce the asphalt of the driveway. The foliage invades the sandbox in the yard. If the roots were left to their own devices, they could damage the foundations of the shed or the sanitary system. “It’s very damaging for homes,” summarizes the project manager of the local ZIP committee.
This case of Japanese knotweed invasion is far from unique. “It is the most reported invasive species to the government. And by far ! It has spread a lot in recent years,” confirms Marie-Ève Tousignant, biologist at the Ministry of the Environment.
Knotweed has already colonized “all the southern regions of Quebec”, from Gatineau to the Magdalen Islands via national parks. It is found everywhere along the Saint-Charles, Chaudière, L’Assomption and Etchemin rivers. Its growth “can be exponential”, since it is easily disseminated by the current of rivers.
No regulation prevents it from being sold in Quebec, laments Marie-Ève Tousignant, even if Japanese knotweed stifles biodiversity, contributes to shoreline erosion and causes many headaches for owners.
The “perfect plant”
It really has the characteristics of “the perfect plant”, confirms Claude Lavoie, a Quebec biologist who has studied the threat. “It’s not killable!” »
With its dense foliage, its stems 3 or 4 meters high, its growth of 4 cm per day, its roots 10 meters long, Japanese knotweed secretes toxins that suffocate competing plants: this fake bamboo really has it all. him.
What’s more, this exotic plant is pretty. This is also why we find it everywhere in Quebec despite its Asian origins. Its beautiful leaves, its white flowers and its exotic side enabled it to win several European horticultural prizes during the 19th century.e century. From then on, it began to be sold in nurseries all over the world. It was still found on shelves on the North Shore until the 2000s, according to Professor Lavoie.
Pretty, but uncontrollable. Human negligence is still largely responsible for its proliferation today. The Ministry of the Environment is therefore betting on a “reduction of its spread”, failing to be able to eradicate it. “People mow the stems and carry these residues to the end of the row or to the bottom of their yard. It takes a piece of barely one gram for the plant to grow back,” explains Marie-Ève Tousignant.
Hence the importance of throwing all knotweed residue in the trash and never considering it as compost.
Scattered eradication attempts
Japanese knotweed is not an ecological disaster like certain forest diseases imported from overseas. But it may only be a matter of time before she does. The colonization of knotweed in Europe is reaching dizzying proportions. It is ranked there as the most expensive plant for society. The bill for infrastructure damage and its eradication is C$425 million a year for Britain alone.
To avoid getting there, attempts at elimination have multiplied recently on our side of the Atlantic.
For example, Michaël Leblanc, director of the Corporation du bassin de la Jacques-Cartier, in the national capital, is working on a “control plan modified year after year”.
“When the colonies are taken in hand quickly, it is possible to uproot week after week and eradicate the plant after 3 or 4 years. But it takes repeated effort. It’s best when the colony is relatively young and weak,” he says.
It takes at least a decade of work to overcome the worst colonies, according to the various experts consulted by The duty.
Drenching plants with glyphosate in the fall is the most effective solution. “The herbicide is the crushing blow, but it’s never the coup de grace”, however nuance Claude Lavoie.
An excellent way to fight against this scourge, unfortunately rooted in Quebec, is to “leave your shores intact”, finally advises the professor. “I have examples of extraordinary rivers in an agricultural environment where the riparian strip has been protected for several meters by allowing shrubs to grow. They have never had an invasive plant problem. They are not able to compete with established vegetation. »
Otherwise, you have to fall back on the uprooting and the installation of tarps to deprive it of sun and water, like what the Montreal team of Valérie Tchang does. “We are finishing a project that we started in 2017,” she said. We succeeded in eradicating it… at 95%. »