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Municipal foresters in Halifax are battling against a fast-growing and hardy weed known as Japanese knotweed that has overrun Point Pleasant Park. ((CBC))

Municipal foresters in Halifax are struggling against a persistent weed they say is threatening to overrun local plants in Point Pleasant Park.

Fallopia japonica, or Japanese knotweed, is a tough, bamboo-like plant that grows to three metres tall, with a root system as deep as seven metres underground.

Authorities are trying an eco-friendly pilot project to eradicate the weed in areas of the park. But they say their efforts to smother the plant by cutting off its light supply have been only partially successful.

"It's an extremely invasive plant," said urban forester John Simmons. "It has the potential to spread and out-compete other plant material. It is a perennial, so it will come back year after year."

This is the third year that Halifax Regional Municipality officials have been monitoring two test areas in the park. A mix of black sheeting and cardboard to block out light and ultraviolet rays at a site on Point Pleasant Drive seems to be showing good results, Simmons said.

But he said a site near the lower parking lot shows evidence of the plants' hardiness.

Simmons said the city is trying not to use chemicals to eradicate the plants. But he admits that while physically covering the plants with cardboard and other materials has the least environmental impact, it might not be to the esthetic tastes of every homeowner.

Halifax landscape designer Linda Van Vulpen said she frequently comes across knotweed in gardens and is often forced to design around the weed, unless homeowners are committed to vigilant maintenance.

"You've got to be diligently looking for new shoots coming out of the ground and continuously plucking them away to prevent them from becoming a strong plant again, because you can weaken it, but it's very hard to get rid of," she said

Simmons said one of largest known stands of knotweed in the Halifax region is on McNab's Island, where nearly an acre is growing unchecked.