Alaska experts try to untangle invasive Japanese knotweed

Japanese knotweed
A growth of Japanese knotweed in Ohio’s Cuyahoga Valley National Park. (From National Park Service)

It comes from Japan, and it can’t be stopped. You can’t burn it, drown it or bury it. Concrete is nothing but so much dust in its path. What is it?

You might guess Godzilla, but it’s even worse. It’s Japanese knotweed, and it’s been quietly taking over many roadways in Southeast Alaska.

John Hudson, with the Southeast Alaska Watershed Coalition in Juneau, discussed the challenges knotweed poses at the Alaska Invasive Species Partnership conference in Sitka in November.

“This plant possesses supernatural qualities, unearthly qualities that are not of this world,” he said.

Hudson is working on ways to destroy a plant that is almost indestructible.

“Apparently you can douse it with salt water; it doesn’t suffer too much from that,” he said. “You can take the cuttings and toss it in the ocean for a couple of days and they’ll just wash up somewhere else and take root. If you’re crazy enough to bury this stuff, dig the hole 15 feet deep — deeper because it can remain dormant underground for 20 years.”

Hudson says it is the most invasive plant in Alaska, and it’s often called the most invasive plant in the world. Like Godzilla, it appears bent on destroying the civilization that unleashed it.

“The freakin’ thing grows through buildings, brick buildings,” he said. “Look at that. This plant grew through a big brick building out the roof.”

The picture Hudson used to illustrate knotweed growing through a building was captioned, “Relax! This is in Britain.” In Alaska, knotweed prefers not the urban zone, but the urban fringe, especially habitat where the much-loved salmonberry grows, and which it will eventually smother. There are about 400 knotweed infestations in Juneau, and a bit less in Sitka, but only because Sitka’s road system is smaller. The infestation is only partly the plant’s fault: The reason knotweed loves highway pullouts and remote turnarounds is because that’s where people put it.

Hudson calls it “Dump Your Stuff Sunday,” an American tradition to discreetly drive out the road somewhere on the weekend and dump old appliances, furniture, and sometimes yard waste. He put up another picture of a pile of dead leaves and branches someone had thrown out beside the road in Juneau. Among all the brown stems were two that were bright green.

a volunteer
A volunteer removes a Japanese knotweed plant (From National Park Service)

“Well, if you look closely at that yard waste pile, yeah, this look appears to be viable stocks of knotweed there — and it doesn’t care that it’s fall, or that winter’s coming,” he said. “It’ll survive the winter. And two years later, exactly. there you have it: a nice little knotweed patch, it will double the next year, double again the next year, keep doubling. And at some point in time, a hydro axe or a mowing unit or a snowplow will come along and it will move it — and that’s where the rest of those knotweed infestations come from.”

No one is quite sure how knotweed came to Alaska. As an aesthetically beautiful plant it almost certainly arrived as an ornamental, probably brought by a gardener rather than a gold miner or, as is rumored, by famed naturalist John Muir.

However it arrived, Hudson is committed to ousting it. He’s found a recipe of 2% Roundup Custom herbicide and 1% surfactant, sprayed directly on foliage late in the growing season, will knock down knotweed — but only if you diligently reapply it annually for several years. He’s been lucky to win the help of the state Department of Transportation with this work, and he soon may have another colleague.

Oregon State University researcher Fritzi Grevstad traveled to Japan to find knotweed’s natural enemy.

“And so this is a picture from from Japan, and this is kind of — often knotweed looks like this, it’s very, very shabby looking,” Grevstad said. “And that’s because there’s a variety of insects feeding on it, and pathogens.”

Knotweed in Japan is engaged in an eternal struggle with a small insect – a psyllid (Aphalar itadori) – which evolved to eat it specifically. Grevstad joined an international team that traveled the length of Japan in 2007 collecting psyllids feeding on all three varieties of knotweed, and brought them back for testing against a number of native North American species related to knotweed. After nine years of testing, the U.S. Fish & Wildlife Service issued a finding of no significant impact from the insects and Grevstad was granted a release permit, giving humans a tiny ally in the battle against this invasive.

“They’re very effective at damaging and killing knotweed,” Grevstad said.

But it doesn’t necessarily mean the knotweed problem is solved. Grevstad has been developing a biocontrol program in the Pacific Northwest, releasing thousands of psyllids in eight states over the last three years, and what worked pretty well in the greenhouse under laboratory conditions has not been as successful in the field, with populations of psyllids failing to survive beyond a couple of winters. Predation by North American critters – like ants – may prevent psyllids from surviving, or other factors that may or may not be present in Alaska. Grevstad says she doesn’t know what would happen if psyllids were released in the Last Frontier.

“They may like this environment better,” she said.

Even if biocontrol works, Hudson warned the members of the Alaska Invasive Species Partnership to never let down their guard – against knotweed, or whatever comes next.

“Spoiler alert here: everybody’s aware of that capsule of soil from an asteroid NASA’s sitting on, that they haven’t quite opened it up yet. You’ve heard of this?” Hudson said jokingly. “They collected soil from an asteroid 100 million miles away. I know what’s in it.”

Robert Woolsey is the news director at KCAW in Sitka.

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