Aggressive Japanese knotweed requires persistence to eradicate

Last Updated: 
July 1, 2008

CORVALLIS, Ore. - Most destructively invasive weeds didn't start out that way. Typically, they developed in an area where, over a long period of time, other plants or animals or environmental conditions kept them in balance with the rest of the landscape.

But in a new setting, without those natural constraints, they can sometimes disperse uncontrollably and muscle their way to complete dominance, crowding out native plants to form a dense monoculture.

Originally imported as a garden ornamental into North America from Asia in the 19th century, Japanese knotweed has become a botanical bully in North America. Together with its close relatives giant knotweed and Himalayan knotweed, these species form a complex of unwelcome intruders that can move in and take over.

"Japanese knotweed has become a nightmare along our waterways in western Oregon, Washington and British Columbia," explained Chip Bubl, Oregon State University Extension Service horticulturist and staff chair in Columbia County. "It spreads by root pieces or green stalks that have been cut and discarded. I've even seen beaver moving stems onto their dams where they sprout more knotweed."

Japanese knotweed (Polygonum cuspidatum) is an herbaceous perennial that grows to heights of eight to 10 feet. Its early spring shoots look a bit like asparagus (and are actually edible.) At full growth, the stalks become woody and look like thin bamboo. It has profuse foliage with many stems and oval-shaped leaves in varying shades of green. The late summer flowers are showy white panicles. These are followed by attractive seed sheathes that are greenish-white and papery.

The plant tends to show up in disturbed areas, such as roadsides, old homesteads and vacant lots. It's also well adapted to stream banks, where the current carries root pieces downstream to spread the infestation.

Getting rid of knotweed is not easy and requires a persistent effort over a period of time. If you leave any living root pieces at all, it springs back with vigor. The non-chemical approach combines cutting, covering and sometimes, digging.

The first step is to cut an existing stand to the ground, being careful to remove all the woody stems, which become sharp-pointed stubs when they're cut. If done in the winter, the stems are dead and pose no threat to sprout. If you want to attempt cutting it during the growing season, when the plant has green shoots, remove all the cut vegetation and bag or burn it. It's definitely not suitable for composting, which has been responsible for inadvertently spreading it to new areas.

Next, cover the area with one or more very heavy plastic tarps or landscape fabrics. Double check first to be sure there are no leftover pointed stubs to puncture the tarps. Take care to extend the tarps at least 10 feet beyond the cleared area and to overlap the tarps by at least a foot. Weight the edges and overlaps securely with rocks or other weights. Knotweed rhizomes are aggressive growers and will seek out gaps or edges even at some distance. If you do this step in the fall, you can be sure it's done in time to thwart the new spring stalks. If you don't want to look at the tarp all winter, you can cover it with bark mulch. Be careful along stream banks as high winter water will dislodge the tarps and send them downstream.

The third step (optional) is to dig up the rhizomes (underground shoots), the main way knotweed spreads. A new plant can develop from even a tiny piece of root, warns Bubl. Rhizome clusters can be large, as much as a foot across, and can send out runners for many feet. Like the cut vegetation, rhizomes should be shredded or burned, definitely not composted.

"Repeat this process each year, without taking vacations, and you have a chance of controlling knotweed in this manner," says Bubl.

If you choose to spray a mature knotweed, it's most effectively done in the fall, when the plant is sending food to the rhizomes for winter storage. Glyphosate herbicides have had good results, especially if you get good coverage of the leaves in the stand. This can be a challenge if the plants are 10 feet tall. Ladders help when spraying vigorous stands. Usually, there will be some re-growth the following year and possibly for several years afterwards. These need to be treated as well. Follow the label directions when using any herbicide.

People do eat young knotweed stalks in the spring. You can see recipes, as well as good photographs of knotweed, on Wildman Steve Brill's website. However, this plant grows and spreads too quickly for eating to be an effective means of control.

Japanese knotweed is listed as a noxious weed in Oregon, Washington, and California and as a species of concern in Idaho. This means that propagation, transport and sale of the plant are prohibited.

Author: Davi Richards
Source: Chip Bubl