Noxious knotweed resists conventional management techniques

Himalayan knotweed. (Photo by Robert Emanuel.)
Giant knotweed infestation. (Photo by Robert Emanuel.)
Last Updated: 
July 8, 2011

CORVALLIS, Ore. – If ever a case needs to be made against introducing non-native plants to a new area, "woody knotweed" says it all. Like a sci-fi beast from outer space, woody knotweed seemingly wants to take over the Earth.

Imagine a weed that you can't pull, cut or mow because you'll encourage denser new growth.

Woody knotweed reproduces itself readily from small pieces of its own roots and stems and can contaminate the soil in which it grows. Knotweed-infested soil is considered a hazardous material in the United Kingdom.

A new Oregon State University Extension publication called "Biology and Management of Knotweeds in Oregon: A Guide for Gardeners and Small-Acreage Landowners" explains that the plant cannot be reasonably managed by non-chemical means. The publication is online and free of charge.

"Knotweed is a huge issue in Oregon and Washington," said Andrew Hulting, weed management specialist with OSU Extension Service and co-author with Robert Emanuel of Oregon Sea Grant Extension and Rebecca Koepke-Hill of the OSU Crop and Soil Science Department.

The publication aims to educate residents who are uncomfortable or opposed to herbicide use, Emanuel said.

"We want to give them the best research-based information so that weed managers, property owners, cities and counties can take the most cost-effective steps to address the infestations," Emanuel said. "To be effective, timing, choosing the best herbicide and application rates are critical."

The best time to treat knotweed with an herbicide is in the fall, from August to October.

Knotweed is the worst weed management problem in coastal counties, according to the number of phone calls and visits to OSU Extension offices and soil and water conservation districts. "For example, in the Nehalem River watershed, one of the largest on the coast, knotweed covers a sizable chunk of the riparian forests and gravel bars," Emanuel said. "In some places at least three species coexist within a few feet of each other and dominate the stream banks."

Woody knotweed is present in almost all Oregon counties. "It is widespread and mostly a riparian weed," Hulting said, "But it shows up in weird upland sites once in a while as an escaped ornamental in yards, driveways and old building sites."

Woody knotweed's large rhizomes (underground horizontal roots) can puncture asphalt, concrete and brick to reach sunlight. The plant canes can be mistaken for bamboo and grow to at least 10 feet tall, with leaves up to 2.5 feet long. It's becoming a serious issue for maintenance of roads.

The four closely related species of woody knotweeds are Japanese, Bohemian, Giant and Himalayan. Photos in the publication identify them.

Strange as it may seem, in their native habitats in Asia, woody knotweeds do not spread aggressively and eventually die back and allow other plants to establish.

"In North America," Emanuel said, "woody knotweeds have a tremendous ability to upset the nutrient cycle in streams and endanger food webs on which salmon depend. That's why control of knotweed is urgent and a priority above other weeds."

Author: Judy Scott
Source: Robert Emanuel, Andrew Hulting