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MASTER GARDENERS: 'Knot' a great gift

(Minnesota Department of Agriculture photo)

A number of years ago, when we were establishing our gardens, a friend made us a gift of "a lovely plant that you will just love." Well, it was lovely and grew rapidly to fill an area that needed a vigorous plant. Then it continued to grow—and grow—and grow some more, under the driveway, up the trees and into the yard.

The plant is called Japanese knotweed or Mexican bamboo (Polygonum cuspidatum) and is native to Asia. Now on the Minnesota Department of Agriculture list of invasive plants, it is extremely difficult to control. In 2006, it was on the list of invasive plants in 25 states and has been introduced in all but eight states.

Keep in mind that there are a number of other plants called knotweed. Common knotweed is a low growing weed of waste areas and yards. Smartweed is a common garden weed but is also called knotweed. Buckwheat, called knotweed by some, is descended from European plants grown for food and bee pasture.

In all there are nearly 1,200 species of these commonly named plants of the buckwheat family known as Polygonum. Japanese knotweed is the largest, often reaching 10 feet tall and spreading by underground rhizomes that reach 5 feet from the original plant—and that's just the first year! It has tall, reddish, hollow stems and large heart-shaped leaves.

In wild areas the plant can displace native vegetation, choke waterways and contribute to erosion by shading out any low growing plants that stabilize slopes and riverbanks. A recent article in "Yard and Garden News" from the University of Minnesota Extension also states that it can damage roadways by growing through cracks.

Eradication of the plant will likely take several years, it certainly did in our case. Start by mowing as much of the plant as possible, once a week is recommended. Next, dig up as much of the rhizome as possible. When shoots emerge, keep digging. Covering the plant with black plastic after mowing is sometimes effective but it is noted that the plant can grow through the plastic.

Be aware that mowing and cutting it can result in small pieces of the root getting spread to other areas where it will root and start a new colony. Sanitation is important. Bag up the cut up plants and dispose of them where they cannot take root. It may be necessary to use chemical control in addition to mechanical control.

A mutual gardening friend solved the problem by enlisting the aid of several football players from BSU to dig up and tackle this nasty plant.

If you want more information on Japanese knotweed, visit the Minnesota Department of Agriculture website, http://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist....

is listed as a specially regulated plant and they recommend eradicating it if it is on your property. This is the best site.

The University of Minnesota has revamped their website: https://extension.umn.edu/yard-and-garden. Go to "Learn About" and find the yard and garden section. There you will find specific areas of information and the "Yard and Garden News" with topics of current interest written in an easy-to-understand format.

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