Introduced weed a knotty problem

  • Jul. 15, 2005 10:00 a.m.

By Heather Ramsay-To some they are pretty flowers for the garden, but to others they are invasive weeds choking out native plants.
Even Mike Cheney can agree Japanese and Himalayan knotweed are beautiful plants, but that won’t stop him from experimenting with ways to kill what is “growing like a weed” on the islands.
The plant is already a huge issue in Great Britian, where it has been busting through pavement and even into Welsh stone masonry homes, and laws have been passed which prohibit the introduction or movement of Japanese knotweed.
Mr. Cheney, who is working on contract with the Ministry of Forests and the Northwest Invasive Plant Council, says although there are many invasive plants on the islands, this is one he feels he may have success with.
He is experimenting with a few different methods of killing off the weed.
One involves spraying patches with salt water at regular intervals and the other involves covering the plant with black plastic and wood chips to keep out the sunlight.
He says world experts are skeptical of his experiment with salt water, but he is finding it kills the leaves of the plant. With successive sprays, the plant is unable to photosynthesize and is weakened, he says.
“If a plant can’t put out leaves to collect light, it won’t survive,” says Mr. Cheney.
His experiment covering the plants with plastic and sawdust has a similar effect, but works better on the Himalayan variety, he says.
He doesn’t believe chemical pesticides are an effective method to control the plant as it regenerates so easily.
“It’s an all or nothing thing. If you don’t get everything [the plant] will grow again,” says Mr. Cheney.
It is also important not to use a blade to tear at these patches, he says, because just one gram of the root or stem can set the plant off again.
“If it ends up travelling on a hoe and lands in the right growing condition, you could have a major patch again after four years,” says Mr. Cheney.
Japanese knotweed, also known as Mexican bamboo or wild rhubarb, is a pioneer species in volcanic areas, breaking up rock so other plants can take root.
“Imagine how tough it is if it drills holes in lava,” he says.
He is trying to deal with patches accessible to humans first, such as on roadsides or in public fields.
Another species Mr. Cheney is concerned about is gorse, a spiny Mediterranean shrub he believes was first introduced on Lina Island. The plant has now spread to the surrounding islands, including Maude Island. He’s also found it on Kumdis Island and in a field across from SuperValu in Sandspit.
Once it gets started, this plant grows high very quickly, then falls and takes root where the branches touch the ground.
Marsh thistle is another concern, but as far as he knows there is only one site on the islands, which he is planning to treat.
If all other eradication methods fail, check out the recipe for apple and knotweed pie at