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The Monday Garden
Invaders: Japanese Knotweed

Issue No. 110 - May 2, 2004
by Sue Sweeney

Out of 4000 invasive plants in North America polygonum cusipdatum has distinguished itself by making everyone’s top 10 list (except you lucky people in Zone 8 and 9; I believe that it also shuns the North Pole). Japanese knotweed, also known as Mexican Bamboo and by several unprintable epithets, is about as stoppable as a major mudslide.

picture: JKW doing a home invasion, 3rd Street, Stamford CT. Spring 2004

A knotweed, JKW’s related to jewelweed see Issue 81 buckwheat, etc. It got into North American in the late 1800’s by use of its good looks and hardiness in the garden. The havoc that it’s wrecking in our wild lands makes you think twice about any alien plant that makes seeds or fruit and is promoted as “fast growing, hardy, easy care, drought and pest resistant.”
Members of The Monday Garden “Eat An Invader Today” Club should note that the young shoots are delicious cooked and served like asparagus. (Caution: Never eat plants grown near a road, building, dumping site or driveway.) However, the yummy shoots are supported by a system of underground stems (rhizomes) that can extend out 60’ from the parent plant, sending up little JKW’s all across your lawn or favorite woodland stream bank. Its need for a root barrier (cage?) is similar to that of bamboo.

Unfortunately, the seeds, which mature in autumn, are spread long distances by the wind and don’t have little root-cages attached. So it’s germinate, colonize, make seeds, get wind borne, germinate, colonize, make seeds. The rapidly spreading JKW stands are so thick that nothing else has a chance. Soon, a colonized area looks like this:


picture: path along the Mill River, Stamford CT, taken over by JKW.


picture: JKW in bloom, Scalzi Park River Walk, Stamford, Ct, Summer 2003


picture: JKW fall color at Scalzi Park River Walk, Stamford, CT, Fall 2003

This no noxious lawn-weed; it’s a certified invader of our precious wild lands.


• First, don’t buy plants that can become invasive, and don’t harbor them in your yard.

• Next, exercise vigilance: learn to identify the bad guys and get the babies before they get established.

• Third, contain existing colonies by preventing further root spread and by keeping them cut back to prevent seed formation.

Yeah, but how do you then get rid of a colony like the one in the top picture? A very small clump can be dug up BUT if you don’t get every bit of root, each bit will become a new plant. Also, when you disturb the soil, all the buried seeds get a chance to germinate. There was an army like this way back in ancient myths: slay one solider and a hundred new ones spring up.

So, what to do? Well, the traditional remedy, short of chemical warfare (which isn’t all that effective against JKW anyway), has been to repeatedly cut JKW to the ground. However, this can take years, and the JKW can probably out last you. Donna Ellis from UConn’s Department of Plant Science, who’s a member of the Connecticut Invasive Plant Working Group had an interesting recommendation for my at Master Gardeners’ program . The new suggestion: cut the JKW to the ground in the spring and cover the area with black plastic or shade clothe for the season. The sun heats the plastic and kills everything underneath, including the JKW’s roots and seeds. The following year, you can replant the area , hopefully with non-invasive natives that provide wildlife food and habitat.


Pictures: Japanese knotweed leaves, 3rd Street, Stamford, Spring 2004


Picture: Japanese knotweed’s distinctive stem, 3rd Street, Stamford, Spring 2003/


picture: dense thicket along path by Mill River summer 2003


picture: winter view of weeds along the Mill River Winter 2003


picture: detail of Japanese knotweed seeds , Mill River, Stamford CT Fall 2003

From The Monday Garden. Copyright © by Sue Sweeney. Reproduced with permission. 

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