Knotweed’s spread has conservationists reaching for herbicide

Dana Coggon stabs an injection gun’s barrel into the stalk of a knotweed, pulls the trigger and gives the invasive plant a dose of blue-hued poison.

“Knotweed is a cancer on our land,” said the Kitsap County noxious weed coordinator. “Unfortunately, sometimes you have to use a little chemotherapy to get rid of cancer.”

Pulling at knotweed is futile. Hacking at it makes it worse. Mowing it spreads its growth, and digging at its roots, Coggon said, “just makes it angry.”

The aggressive Asian import has swallowed up parklands, roadsides and crept onto stream banks, where native plants and even salmon are crowded out.

The county Noxious Weed Control Board and the Kitsap Conservation District are upping the ante in their fight against knotweed with an expanded chemical spray program targeting swaths of knotweed on Bainbridge Island.

Coggon holsters her injection gun after workers rigged with pack-mounted tanks and sprayers doused thick stands of the bamboo-like weed along Lovgren Road, turning broad green leaves and white flowers a light shade of blue.

While the injection method was once preferred, spraying saves time and uses less herbicide per plant, according to conservation district director Brian Stahl.

The Lovgren site was one of about 200 on the island slated for herbicide spraying this week. About two-thirds of the sites are on city and park district-owned properties. The remaining spray zones are on private parcels at the invitation of owners.

Funded in part by a $9,000 state Department of Agriculture grant, the knotweed eradication program is the largest of its kind in the county. The grant specifically targets Bainbridge because the island has undertaken significant knotweed control measures in the past.

“Unfortunately, we don’t have the funding to do anything remotely like this anywhere else in the county,” Coggon said.

Hesitant to use herbicides that can harm the environment, Bainbridge officials have long used non-chemical means to halt the spread of invasive plants. In the case of knotweed, the earth-friendly approach has had unintended consequences.

“Everything on the roadside was just mowed by maintenance workers,” said Coggon. “So that means it just kept spreading, and mowing made it worse.”

Knotweed’s ability to sprout new plants from cut fragments allowed it to spread at a more rapid rate with each mowing. Fragments caught in mowing machinery helped disperse the plant to new territory.

Unlike some invasive plants – such as the Himalayan blackberry and English ivy – knotweed has made relatively recent incursions onto the island. But after a few decades, the plant now dominates large portions of Port Madison, Hidden Cove and Pritchard Park, as well as numerous roadsides.

The city in 2006 amended a ban on the use of the herbicide glyphosate, a chemical considered toxic to humans and animals, for controlled use against knotweed on city property. Initially limited to injections into individual knotweed stems, the herbicide policy was amended again last year to allow limited spraying.

In most cases, spraying will actually introduce fewer chemicals into the environment than the injection method, according to Coggon.

Each stem injection used up to 7 milliliters of herbicide. But spread over stands that could include 300 to 3,000 plants, the herbicide could exceed legal limits. The county’s spraying methods, on the other hand, use a 3.5 percent herbicide solution per gallon of water. A typical stand of sprayed knotweed would receive about one-tenth of herbicide as an injected stand, Coggon said.

Traces of the herbicide disappear after about a month.

“Once it hits the soil, microbes break it down and it’s gone within 30 days,” she said.
No spray is used in “ecologically-sensitive areas,” including stream or wetland zones, Coggon added.

“In those areas, injection is the best method because it keeps the herbicide in the plant,” she said.
Sprayed areas were marked with a blue dye mixed with water and herbicide. Warning signs were also posted.

For Dale Spoor, who led efforts to ban herbicides on school, park and city property, a little poison is a necessary evil when combating knotweed.

“We’re faced with several bad alternatives, so we’re forced to choose the least destructive,” he said. “Knotweed is really, really rapacious. It spreads rapidly and quickly displaces native vegetation, so I don’t think we have any choice but to try to control it.”

A member of the buckwheat family, knotweed was imported from Asia to spice up home gardens on the East Coast. As late as the 1950s, the Sears catalog sold knotweed as an ornamental plant.

Gardeners began to realize that the plant not only thrived in North America – it dominated.

“Some people on Bainbridge have, for 10 or 15 years, been digging at small patches of knotweed in their yards,” Coggon said. “And they’re still digging.”

Like mowing, digging creates knotweed fragments, and each fragment sprouts a new plant.
With roots that can spread up to 65 feet, knotweed will often tunnel under paved roads to fresh territory on the other side.

Once it reaches stream banks – its preferred habitat – knotweed can grow with such density that salmon and other wildlife are pushed out, according to Jeanette Franks, founder of Weed Warriors, a volunteer invasives removal group.

“In general, I never advocate using herbicides,” Franks said. “However, with knotweed it’s the only thing that works.”

For more information on the Kitsap County Noxious Weed Control Board’s knotweed eradication efforts, click here.

Knockout knotweed in your backyard
It takes a good dose of diligence and vigilance to combat the knotweed invasion. Homeowners can help in the overall effort by tackling the invasive weed in their yards. The first rule is not to mow knotweed. This only encourages the spread of knotweed as each fragment will sprout a new plant. Instead, carefully cut each stalk and pile them on a tarp to dry out. If you’re not using herbicides, you’ll have to keep at it, cutting twice a month from April to the first frost over the course of two to three years.

2 thoughts on “Knotweed’s spread has conservationists reaching for herbicide

  1. Where’s the outrage? We’re talking chemicals here. Aren’t all chemicals poison? Isn’t all poison bad? And who said these county people could bring their poisons onto our beloved island? Don’t they know we’re really part of King County, well Seattle, really? Don’t they?

    OK, so organic chemicals must be good, because they are organic and probably sustainable, but that’s not the point.

    People who chop, dig, mow or poison knotweed are just SELFISH! Knotweed is a living thing. It must be nurtured, cared for, understood. Salmon and knotweed must learn to get along. Let’s have a city sponsored charrette, hire some consultants, sell some councilmanic bonds.

    There must be solutions other than killing living things.

  2. “Pulling at knotweed is futile. Hacking at it makes it worse. Mowing it spreads its growth, and digging at its roots, Coggon said, “just makes it angry.”

    I wonder what happens when you throw money at it.

    KC’s noxious weed program is lobbying for more money. Hopefully, if they get it, they won’t do what Kitsap County has traditionally done with any increase: hire more staff.

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