Urban Forest Ecology: Knotweed with Laura Wyeth

Laura with Knotweed
Council member and horticulturist Laura Wyeth, with Japanese knotweed. Photo by Larry Decker

Japanese knotweed (Polygonum cuspidatum; Latin synonym Fallopia japonica) is indisputably a major nuisance in the urban forest. NYC Parks Natural Resources Group has documented extensive research and control methods they’ve used, in partnership with the Bronx River Alliance and other entities, to combat knotweed along the Bronx River and in other natural areas in the City’s five boroughs.

Regardless of what combination of chemical and mechanical means are employed, control of Japanese knotweed is widely acknowledged to be imperfect at best. While we can and should manage its presence in our urban forests, this plant is here to stay.

Why and how is Japanese knotweed so successful at colonizing the most degraded sites? Why is it so hard to control? And what are some of the positive attributes of the plant that you might not hear much about? In this piece, Council member Laura Wyeth, a horticulturist with a particular interest in weed science, explores some of what makes Japanese knotweed unique.

Laura Wyeth:

Japanese knotweed is a plant caught up in the polarization of our times. Both its virtues and its malevolence have been inflated in the public imagination, and it is at once known as both miracle drug and botanical monster. In Canada, a nation not prone to paranoia, it has been described as “a demon weed so relentless, bloody-minded and destructive, it’s been called the terrorist of the plant kingdom.” Strong words for a creature with no teeth, claws, or toxicity to humans. Internet herbalists tout its miracle cancer-fighting properties with equal hyperbole. Yet with this plant, the quiet facts are more interesting than the hype.

Brought to Great Britain and the U.S. from its native Japan as an ornamental perennial in the late 19th century, Japanese knotweed behaved itself in backyards and botanical gardens until about the 1930s, when it  escaped the garden gate and set about colonizing. It now grows just as happily in roadside drainage ditches and construction dumpsites as it does along woodland margins and streambanks.

Japanese_Knotweed
Japanese knotweed flowers are used by bees for nectar, producing a dark “bamboo honey” considered high in quality. by Jan Samanek, State Phytosanitary Association, bugwood. org

The plant’s hollow, arching canes can grow 8 cm in a day. Its horizontal rhizomes send out runner shoots in all directions so that, like bamboo, an individual knotweed plant can quickly grow into a dense thicket. Its tissues have remarkable powers of regeneration; a piece of broken stem or rhizome the size of a thimble can resprout into a new plant, and this often happens when soil containing pieces of rhizome are transported. Consequently, knotweed is often seen growing in construction sites where topsoil has been brought in from elsewhere. It soldiers on through crummy soil and can sprout through small pavement cracks or push its way through asphalt.

japanese-knotweed
The bamboo-like stems of Japanese knotweed. Photo by Barbara Tokarska-Guzik, University of Silesia, bugwood.org

Try to dig one out, and three will take its place. Knotweed laughs at pesticides. Its only predators, a few insects and one or two fungal diseases, have not followed it from Japan, and so its spread is unhindered. It is now considered highly invasive throughout the U.S., Canada, and much of Europe.

Japanese knotweed is such an issue in Britain, where it enjoys a longer, milder growing season than in the U.S., that companies in almost every county specialize in trying to eradicate it and an equally numerous class of lawyers practice litigating claims over it. The menacing presence of the plant has caused neighbors to file lawsuits, banks to cancel mortgages, and homes to lose their value. Here in New York State, its ability to edge out native species is considered its greatest destructive power, and, as in Britain, it is illegal to sell or purchase the Japanese knotweed plant.

Eat the Invaders knotweed
Strawberry knotweed pie on the Eat The Invaders website. See also recipes for knotweed wine, knotweed gazpacho, chutney, vodka, fruit leather, pesto …

It is not, however, illegal to eat it. The young shoots of the plant are edible in spring and taste pleasantly tangy, much like its cousin, rhubarb. And like most illicit substances, there’s money to be made in it. The stems and leaves are high in resveratrol, the same compound found, famously, in red wine. Resveratrol has been shown to reduce lipid levels in mice, and some studies suggest that it may have protective effects against cardiovascular and neurological decline, or even anti-cancer properties, though the evidence is murky and has been tainted by laboratory scandals.

Despite the lack of strong evidence, resveratrol has been touted by many writers of health articles and makers of herbal supplements as a red wine-based anti-aging wonder drug. Resveratrol tablets and tinctures are usually marketed with appealing images of grapes and wine, but in fact, the majority of resveratrol supplements on the market are, far more lucratively, isolated from Japanese knotweed.

The plant has its own reasons for making resveratrol—it’s a part of knotweed’s defense system. Resveratrol is a phytoalexin, a compound produced in the tissues of the plant in response to microbial attack. Though the evidence for resveratrol’s effects on human health is unsubstantiated, research in the agricultural realm has found extract of Japanese knotweed to be an effective antifungal agent, protecting crops such as tomatoes, wheat, and cucumbers, from various fungal diseases, most notably powdery mildew, gray mold, and rust. Knotweed’s naturally-induced phytoalexins inhibit the formation of spores in pathogenic fungal species, and the extract, when sprayed on certain crops, confers the same benefit. As it is plant-based and minimally toxic to mammals, it is considered safe for organic farming by the National Organic Program, and is marketed in the U.S. under the trade name Regalia.

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Image by Steve Manning, Invasive Plant Control, bugwood.org

Invading marauder, life-extending miracle powder, literal home-wrecker, organic farmer’s friend … Japanese knotweed deserves one more public reputation—first responder. In its native habitat, the volcanic slopes of Japan, it has long played that role. It is the first plant to appear after the lava has flown; it colonizes the bare rock and slowly builds soil, allowing other species to return. When it appears in habitats disturbed by humans, as it has all over Europe and North America, it is only following its tenacious nature. And for that it deserves, at least, some respect.

Works Cited:

-Bailey, J. P., and A. P. Conolly. “Prize-winners to pariahs-a history of Japanese knotweed sl (Polygonaceae) in the British Isles.” Watsonia 23.1 (2000): 93-110.

-Hammerschmidt, R., and E. K. Dann. “The role of phytoalexins in plant protection.” Insect-Plant Interactions and Induced Plant Defence 223 (1999): 175.

-MacQueen, Ken, “The plant that’s eating B.C.”, Maclean’s. 2015

-Quarles, William. “Giant Knotweed, Plant Disease Protection, and Immortality.” The IPM Practitioner, 31(¾), 2009.1-7.

-Soleas, George J., Eleftherios P. Diamandis, and David M. Goldberg. “Resveratrol: a molecule whose time has come? And gone?.” Clinical Biochemistry 30.2 (1997): 91-113

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