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Once a landscape plant, Japanese Knotwood now worst invasive

Once a landscape plant, Japanese Knotwood now worst invasive

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In several counties here in Western Wisconsin, Master Gardeners have been banding together with others to try to get rid of an invading plant called the “King of Weeds.” That plant is Japanese knotweed. Eradication parties have been organized in many areas with dozens of people trying to clear this herbaceous perennial out of the locality. In fact, Wisconsin and the surrounding states have posted numerous bulletins regarding the plant. In other countries it is considered a controlled waste plant. Heavy spring run-off and flooding have caused new infestations and ecological threats.

In a research paper dated to 1964, Hitchcock and Cronquist flat out state that Japanese knotweed is capable of crowding out any other plant species and taking over. They caution that it was a major mistake to import it in the first place. That was fifty years ago. Knotweed is now so intrusive that last year the World Conservation Union classified it as one of the world’s worst invasives. Major localities of concern are riparian areas. Here in Wisconsin, that makes it a huge problem and also because knotweed contains allelopathic compounds and poisons the surrounding plants, killing them. This disrupts the nutrient cycle of an entire area.

It’s not often that I “blast” a plant. It’s just that this one has been mismanaged so badly that it will take a concerted effort to regain control. No one expected Japanese knotweed to try to take over the world and become a monster. It seems mildly amusing to find “alien” workshops on the plant. We are being attacked, but it isn’t from outer space! The Japanese knotweed problem has gotten so bad that a Newsweek article from July 2014 suggests that the discovery of knotweed on a property can actually decrease the value your home.

Japanese knotweed (Fallopia japonica) is from the family Polygonaceae. It is a native to the Orient. There, it has traditionally been used as a fresh vegetable. Knotweed was brought to this country as a landscape plant. The plant likes all kinds of soil and thrives in almost every type of weather. It can withstand temps to -30 degrees and loves hot, humid weather, as well. It also tolerates both full sun and total shade.

This buckwheat cousin is a large plant with hollow stems or stalks. The stems are smooth, round and have brown blotches on them. They also have nodes that make the plant resemble bamboo. It is sometimes called Mexican bamboo because the stalks grow up to fifteen feet tall. However, it’s not the stems/stalks that are the problem. The roots and rhizomes are strong and can break through concrete, often damaging sidewalks and house foundations. The rhizomes can reach down into the earth as far as ten feet and branch horizontally more than 20 feet from a single plant.

Japanese knotweed is not a small plant. It grows into a dense shrubbery and can form overhead canopies. The stems are smooth and swell at the joint where the leaves attach. Thus, the bamboo reference. The stems also clump and form dense thickets. Prolific does not even seem to cover what knotweed does. The leaves can really be any size, but average six inches by four inches. They are bright green and oval-shaped with a triangular tip.

If the plant is cut down, each rhizome sends up a new stem. If the plant is tilled under, the rhizomes are cut and spread, with each having the capability to grow a new plant. Control seems only to come by burning the entire plant or by cutting the plants down and digging up the roots. Pulling young shoots out by the roots, before they get too large, also works. Otherwise, repeated chemical control seems to be the only answer. The main chemical used seems to be glyphosate, although with the recent controversy over manufacturers, it’s an individual choice which is the lesser of the two evils. Many agricultural and government sources state that it will take several years to divest your property of the plant.

Japanese knotweed was imported because where the leafstalk meets the stem, gorgeous, greenish-white, lacy, spiked flowers form. The flowers attract honey bees, and are prized by apiarists and honey farmers. The tender reddish-purple, mottled green shoots are a component in literally hundreds of foraging recipes. If you do harvest the plant for food, make sure it is pesticide free.

Just like any “weed” there is a value to the plant. Besides an obvious food value, knotweed contains resveratrol, which has been shown to delay the onset of Alzheimer’s. In Chinese medicine, the knotweed root is harvested and dried. The resulting “hu zhang” is used to treat heart, liver and gall bladder problems. In Japan, knotweed is known as “itadori” (pain puller) and the tea made from the root is a powerful anti-inflammatory and pain killer. Besides resveratrol, medicines made from the entire plant have been shown (and used effectively) as a major treatment for Lyme’s disease.

Master Gardener Sydney J. Tanner nurtures her 10 children as well as plants, in Colfax.


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