Some attractive plants are noxious weeds in disguise

Published 5:00 pm Wednesday, September 25, 2002

Sometimes a gardener’s best intentions can go awry. That is certainly the case with several members of the Polygonum family – giant, Japanese and Himalayan knotweed and their innocent introduction years ago by some well-intentioned garden explorer looking to bring new plants to the United States.

Imported from Asia and elsewhere as ornamental plants, the big perennials grow by rhizome and spread in moist areas. Oh, and in some sunny areas, too. The Himalayan is in flower now throughout the North Coast, while the giant variety is more noticeable for its huge, heart-shaped leaves. The knotweeds are on the Clatsop Weed Management Advisory Committee’s noxious weed list and their control will become part of the committee’s comprehensive weed management plan.

Ridding your land or garden of knotweed involves cutting the plant to the ground, and then throwing it away or burning the debris, says Clatsop County Extension Agent Justin Williams. “You also might consider tarping a knotweed patch to keep the sun out,” Williams suggests.

He notes that a herd of Boer goats brought to Astoria by the city and the Clatsop County Soil and Water Conservation program has made “amazing” progress on the Japanese knotweed that covered a hillside near Shively Park. Williams says that he’s observed the hooved helpers choose the knotweed over blackberries.

“Literally in five days the site was down to stubble,” Williams says.

If goats are not an option for you, cut knotweed down frequently to prevent the leaves from taking in enough sunlight to grow the plant’s root system. Don’t put the cuttings near a riverbank or stream. Like the beautiful Frankenstein it is (I admit that the Himalayan’s red-tinged leaves and creamy white flowers are striking) knotweed will respond to the moisture and rise anew. Williams urges people not to use chemical treatments to rid their land of knotweed – because the plant prefers damp soil, there’s a chance the chemicals will run off into our local waterways.

Management of riparian noxious weeds is one topic that will be covered in an upcoming Watershed Stewardship Education Program offered by the Oregon State University Extension Service. The program runs from Oct. 3 to Jan. 11 and provides basic information to help local residents and groups address watershed issues. Participants can attend all or some of the classes, although completion of the program means Master Watershed Steward designation. For more information, call 325-8573.