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Join the battle against knotweed

Join the battle against knotweed

P 2 A knotweed pic

Jennifer Grenz, described as the foremost expert on knotweed for BC’s west coast, outlined what to do and what not to do when dealing with knotweed. Donna McMahon photo

Collective effort is needed to halt the spread of knotweed on the Sunshine Coast, says knotweed expert Jennifer Grenz, who addressed a public information session on June 22 at the Sechelt Band Hall organized by the Halfmoon Citizens’ Association. The meeting focused on techniques for controlling knotweed, and especially what property owners should NOT do if they discover the weed on their property.

Japanese knotweed is a bamboo-like shrub that grows six to 10 feet tall. There are three types on the Sunshine Coast: regular Japanese knotweed, Giant Knotweed (which grows up to 15 feet tall), and new hybrid species of knotweed which is much more aggressive.

“Now instead of just spreading by plant parts, they also produce viable seed,” said Grenz. She likened knotweed to an iceberg. “What you see is really only the tip of what is actually going on in the ground.” Roots can go down three metres deep and travel 20 metres laterally.

Knotweed grows rapidly into a dense canopy that chokes out every other plant and exudes chemicals into the soil that inhibit the growth of other plant species.

Mowing and weed whacking cause knotweed to spread, because the weed can regrow from small fragments. Trying to dig it out or cover it is also ineffective. Control programs use herbicides, often requiring two to four treatments over several years.

Herbicides can be sprayed or injected, but at present, stem injection is unavailable because Health Canada declared that this “off label” use of pesticide cannot be allowed until studies have been done proving its safety and efficacy. The approval process is underway, but will take time.

Grenz noted that people tend to worry about spraying, but that spraying is often the most efficient treatment. “When we use the stem injector we’re putting out 500 times more product per square metre than when we spray,” she said.

Stem injection is appropriate where spraying cannot be done, such as near water and where knotweed is mixed in with other important plants. Grenz also stressed that herbicide should never be used when bees are on the plants, or where plants are close to berries that people will be picking.

“You need to think before you hit the trigger,” said Grenz, advising homeowners to read labels and be familiar with local bylaws. She urged people with large knotweed patches to call in qualified contractors who have access to more potent products.

For small, isolated patches, manual removal can work. At Henderson Beach in Roberts Creek, a group of 20 volunteers has succeeded in controlling a patch of knotweed. However, it takes dedication. Knotweed sites must be tended regularly for a period of years.

And disposing of knotweed cuttings is also tricky, since green waste facilities won’t take them. At present the best option for coast homeowners is to dry the cuttings out completely and burn them.

Finally, restoration and re-vegetation of treated areas is crucial, said Grenz, and this is where community volunteers can help. She recommended “Grow Me Instead”, a booklet produced by The Invasive Species Council of BC for gardeners and landscapers.

Getting private property owners on board is key, said Grenz. “A lot of people don’t even know they have it. If you know what knotweed is and your neighbour has it, tell them about it.”

Donna McMahon



An earlier version of the story incorrectly stated that “The SCRD is compiling a list of qualified contractors.” However, the SCRD does say that its website “provides a list of tips for property owners interested in getting professional help at”


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