Greasy Creek landowners join effort to dislodge non-native plant

The broad leaves are heart-shaped, squared off at the stem. The flowers grow in clusters, delicate and white. They’re so pretty, a few early residents in the Marys River Watershed proudly planted them as hedges. These well-meaning landowners couldn’t have known back then what biologists know now: This seemingly lovely plant takes over like gangbusters. It chokes off native vegetation, creating a monoculture and severely disrupting the ecology of Pacific Northwest riparian areas.

It’s called knotweed — an apt name for a plant that forms dense systems of roots that, once established, are nearly impossible to eradicate. “Knotweed alters riparian vegetation communities, disrupts nutrient cycling, negates riparian restoration efforts, affects the recreational use of watercourses, and decreases property values,” the Washington State Department of Agriculture (WSDA) explains in a recent report. “These plants are pioneer species that quickly colonize disturbed areas. Once knotweed becomes established, it is very difficult to remove, and single patches can persist for more than 100 years.”

Just ask Bob Lillie and Barbara Weeks. Fifteen years ago when they bought their rustic home in a dappled glen on Wells Creek, an upper tributary of Greasy Creek, a dense hedge of knotweed towered 12 feet high across the front of the property. Someone had even gone to the trouble of planting knotweed inside an old tire as a decorative touch.

“This was a jungle of knotweed,” says Weeks, a retired teacher from Philomath School District. At first, the couple didn’t realize their giant hedge was a giant nuisance. Then they attended a neighborhood meeting at the Philomath Grange Hall where they heard the results of a “rapid bio-assessment” made by the Watershed Council. A snorkel survey of Greasy Creek had turned up patches of Japanese knotweed (one of four species of knotweed) throughout the system.

Lillie and Weeks were alarmed.

“I said, ‘I think we have the mother lode of knotweed on our land,’” Weeks recalls. That was 2011. In 2013, tramping around their acre of woodland, Lillie and Weeks spy only one sprig of the stubborn weed hidden among some ferns. Lillie, an emeritus professor of geography at Oregon State University, also points out a recent sprout popping up in a flowerbed where once the 12-foot hedge loomed. But most of their mother lode is gone.

These two educators were among the first Greasy Creek landowners to join the knotweed control program launched by the Council in 2011. Two and a half years later, 97 percent of the landowners have either allowed the Council to spray their knotweed with herbicides imazapyr and/or glyphosate  (depending on the treatment year and proximity to the creek) or have treated their weeds on their own.

But it will take constant vigilance to keep the knotweed at bay. Riparian planting coordinator Kathleen Westly, who is heading up the Council’s multiyear knotweed control program, recently put on a wetsuit and waded several miles up the creek, sometimes pushing against waist-deep waters, to check for new invasions. She spotted a handful of new plants, which will by treated this September along with the rest of the Greasy Creek basin. Next spring, she and her team will spread weed cloth over the so-called “dead patches,” tacking it down with landscape staples, expecting this approach will minimize the herbicide required for re-treatment in the fall of 2014.

Why bother with dead weeds? “It can look dead,” Westly says. “But if you break off the canes, you can still see a mass of teeny buds ready to leaf out or break off and float downstream.”

The stream’s current is a major vehicle for knotweed invasion, the channel its highway. “In river corridors, knotweed reproduces from fragments and seeds that travel downstream, affecting the gravel bars and riparian forests of entire river systems,” the WSDA cautions. “Root and stem fragments as small as one inch can produce a new plant. As a result, one patch can be the source of many downstream populations.”

Even the most innocent human activity can give the knotweed a new foothold. In the Pacific Northwest, knotweed spreads when roots and stems are moved by flowing water or by humans via lawn mowers, shoes or clothing, the WSDA says.

For more information about the Watershed Council’s knotweed control program, contact Kathleen Westly at [email protected].