Sean C. Morgan
Japanese knotweed is a relatively new problem in Oregon, and it may have been caught early enough to be stopped, unlike its fellow non-native species, the Himalayan blackberry, Scotch broom and English ivy.
Jodi Lemmer, of the Lane Soil and Water Conservation District, and consultant Jeff Jone, who often works within the Siuslaw watershed, presented information on the knotweed and their efforts to control it at the Calapooia Watershed Council’s annual meeting last week.
The Calapooia River Watershed is among those affected by the weed.
Japanese knotweed arrived in Oregon from Asia via England, Lemmer said. At least that’s what experts think at this point.
The weed has heart-shaped, smooth-edged leaves. It blooms in late July and August and grows to five or six feet each year before the stems die back in the fall. Its stems are hollow and bamboo-like. It grows faster than English ivy or blackberries. It often grows in blackberry patches and outperforms even blackberries.
One of the problems with the plant, Lemmer said, is that it is so vigorous it completely chokes out native species.
“Our native species provide a lot of structure to our streams,” Lemmer said.
Native plant species form the base of the food chain in Oregon. Losing them to knotweed destroys fish habitat and bank-holding structure.
Japanese knotweed cannot stabilize banks with its shallow horizontally growing rhizome root system. The rhizome system means that small fragments of roots can sprout into new plants. The roots can spread some 26 feet from the main plant.
When it dies back for the winter, it leaves banks bare, Lemmer said. That’s when winter stream flows erode the banks.
The weed spreads by streams eroding infected banks along with human and animal activity.
It grows well in disturbed soil and can take hold in gravel bars, completely covering them in a matter of three or four years.
Lemmer used an $8,000 grant to find and combat knotweed in the Lost Creek Watershed last year. She presented photos of infestations and her efforts on Lost Creek.
Herbicides are the most effective way to treat knotweed infestations, Lemmer said. Mowing the weed just spreads it. In one photo, she showed how mowing weeds down caused them to invade a man’s front lawn. Pulling them or mowing them requires tenacity ? keeping them pulled constantly and drying them out on a tarp.
In riparian areas, Lemmer recommends using Rodeo or Aquamaster, which have the same herbicide as Roundup, but are allowed for use streamside. The project used both broadcast spraying and injection of herbicide directly into the stems of the plant.
“It’s a pretty new weed,” Lemmer said. “If we can get it early, we think we have a good chance of controlling it. We don’t have the definitive answer yet on what’s the best methods.”
In other business, members of the council re-elected Peter Jensen, Duane Smith, Mike Nehls and Tim Otis to new terms on the steering committee. They elected Frank Lamb to fill Will Carey’s seat.