There are weeds, there are bad weeds, and then there is Japanese knotweed. In its native habitat, knotweed can survive volcanic eruptions. It can lay dormant 10 feet deep for 20 years and then emerge through cracks in lava.

Knotweed spreads rapidly, creating a tall, dense canopy that shades other plants. Whether you’re starting from a lava field or a diverse landscape, the result is the same: a dense monoculture of knotweed.

Japanese knotweed (Polygonum cuspidatum, recently reclassified to Reynoutria japonica) is a distant relative of bamboo. The perennial dies back to the soil surface every fall but grows quickly in the spring from rhizomes (underground bulbous stems, like ginger).

Controlling Japanese knotweed is like battling the Hydra of Greek mythology. Every time one of the Hydra’s many fierce heads is cut off, two heads instantaneously emerge. When a knotweed crown is damaged, such as by cutting, covering or herbicides, the surrounding dormant buds are stimulated. They break their dormancy and grow new shoots, sometimes several feet away from the original crown.

The myths surrounding knotweed are almost as abundant and persistent as the plant. Fortunately, there are ways, even organic ways, to keep this beastly plant in check.

The three main ways to control knotweed are by cutting, covering and chemicals. Digging is an option for a tiny plant, but heavy machinery is needed for mature plants (which require excavating nine feet deep and wide). No matter what approach is used, repeated measures and long-term monitoring are recommended.

Cutting
In the coastal village of Alma in New Brunswick’s Fundy Biosphere Region, a not-for-profit organization has found a way to slowly reclaim land captured by knotweed. Moreover, the solution doesn’t require chemicals or tarps – just hand tools and patience.

Keith MacCallum, executive director of the nonprofit organization Guardian Ecology, along with help from summer students, cuts knotweed using machete-like tools. MacCallum prefers hand tools because they allow him to avoid damaging native plants.

“If you cut knotweed once a season, it encourages growth,” MacCallum explains. Cutting twice will keep it from spreading but cutting it more often can reduce its vigour.

“You’re trying to exhaust the rise of energy stores by forcing the plant to continue to put its energy into growing new shoots.” It is most effective to cut in the spring after the knotweed has depleted some energy reserves over the winter. MacCallum cuts patches at least two or three times before the end of June and two or more times later in the season.

MacCallum recommends that homeowners cut knotweed as often as possible. Vermont researchers found the formula 12-11-10-9-8 was successful with 12 cuts in year one, 11 cuts in year two, etc. After the fifth year, they recommend cutting five to six times a year until the weed is eradicated.

Covering
At Fundy National Park, Neil Vinson is taking a different approach. In 2016, the resource management officer started covering knotweed with heavy-duty landscape fabric. 

He starts with intensive site preparation. Park workers remove the stems and dig out enough of the crowns so tarps can lie flat. The fabric, secured with old lumber, extends a few feet beyond the crowns.

Vinson doesn’t plan to remove the tarp anytime soon. After six years, the knotweed is still alive but considerably weaker. Vinson will soon plant native species by inserting willow or chokecherry stakes through holes melted in the landscape fabric.

Chemicals
The herbicide glyphosate (often sold under the brandname Roundup) is sometimes used to try to control knotweed. The chemical can be sprayed on plants or injected into the stems. Like cutting and covering, using herbicides takes multiple treatments and isn’t always effective. Widespread concerns surround the effects of the chemical on human and environmental health. If you choose to use herbicides, professionals have the training and equipment to reduce environmental exposure.

Since 2019, Tyler Jollimore has tackled invasive weeds using mechanical and chemical control through his Halifax-based business, Knotweed et al. He cuts knotweed to gain access to the canopy. Then he sprays the herbicide glyphosate twice a year. He can kill a small patch in a year, but larger patches may take up to four years.

Knotweed forever?
No matter what control approach is used, the four Ts apply: be timely, tenacious, tough and thorough. When cutting or spraying, repeated measures will be most effective. Even after apparent eradication, the area should be monitored for years afterward.

I started my personal battle with knotweed when I moved to rural New Brunswick 15 years ago. For several years, I cut the plant once or twice every spring, but this had little effect. Finally, I am making progress, thanks to support and guidance from Guardian Ecology over the last two years.

I considered covering knotweed, but I didn’t like the initial cost nor the eventual disposal of old landscape fabric in the waste stream. Herbicides are not an option because I don’t want to use toxic chemicals on my land.

Instead, I am cutting it. I use a battery-powered chainsaw to cut tall stalks and loppers to cut close to the ground.

Knotweed, like rye and yellow sweetclover, may contain natural allelopathic chemicals that inhibit germination of other plants. Using that to my advantage, I mulch with knotweed in my garden pathways (after ensuring cuttings don’t have rhizomes). The mulch controls weeds and adds organic matter. I also compost the stalks in piles where knotweed grows.

I still have knotweed on my property, a lot of it, and doubt I can ever eliminate it, given the considerable size of the patch. But Guardian Ecology has almost eradicated one patch and I’m weakening another. Meanwhile, whenever I want a bit of exercise and some mulching material, I go into my knotweed jungle.

Sterile seeds
“I would gladly deal with Japanese knotweed rather than the slew of other invasive species,” Neil Vinson says. At Fundy National Park, he is also battling woodland angelica and glossy buckthorn. 

One positive trait of Japanese knotweed is that it spreads only by rhizomes, not by seed (although seeds of Japanese-Giant hybrid knotweed are viable). In contrast, other species spread by seed. With the invasive shrub glossy buckthorn, Vinson explains, “the birds eat the berries, fly away and defecate somewhere. Suddenly, you’ve got a new population cropping up somewhere else.

“It’s a lot of work to contain knotweed but, once it’s contained, it’s really not going to go anywhere as long as we’re handling the material appropriately.”

“Knotweed seems impossible to kill,” Vinson adds, “but it seems like as long as we’re treating it appropriately, monitoring it and containing it when we find it, it’s much easier to deal with than these other invasive species.”

Eating your enemy
Jessika Gauvin helps weaken my knotweed by harvesting shoots in May. Jessika describes knotweed as one of her favourite wild foods. That is saying a lot because Jessika says she is “a lifelong forager, a mushroom identification specialist, wild food guide and herbalist.” Through her business, Enchanted Mushroom Forest, she teaches foraging and wildcrafting.

“The flavour of knotweed varies depending on what kind of soil it’s growing in,” she says. “Rocky soil makes for a more savoury, asparagus-like flavour, whereas richer soil results in a sweeter, tangier plant.”

She has made knotweed wine, candy, fruit leather, rhubarb-style crisp, and more. Her favourite preparations are pickles (made by putting peeled shoots in dill pickle brine) and jelly, which has a “sweet and tart flavour and a beautiful, delicate pink colouring.”