Knotweed

Asked May 10, 2016, 11:27 AM EDT

Hello, I have Japanese knotweed surrounding two sides of my yard. Most of the knotweed is on a slope of Pgh public property. The rest is in a neighbor's yard, and they say they have no time to deal with it, and I get that. I'm interested in gardening a barrier that also helps with sound (since 376 is also near by). My best idea now is to plant a row of Leyland Cypresses and then some ferns and other strong bushes as a sound & gardening barrier to hopefully help prevent the knotweed from taking over. and moving too far into my yard. It feels like it creeps more and more every week. I tried digging out most of its roots last year, but that was too time intensive and it all came back, so now I just pick it and throw it away when I see it. I was thinking of using landscaping cloth and mulch to really help drive it all home. So - what do you think of my plan? Can I plant all those things through the landscaping cloth? Will that just make the knotweed go under the cloth and further into my yard??? Is the landscaping cloth necessary? OR is mulching good enough. I know it will keep coming back, but there are some places where there is jewel weeds and other plants and that seems to lessen the knot weed, so I figure gardening is my best bet. Any other ideas and suggestions would be much appreciated, as well as specific feedback on my idea. Also, I am against roundup or any non-organic remedies. Thank you!

Allegheny County Pennsylvania japanese knotweed japanese knotweed control

3 Responses

Unfortunately Japanese knotweed spreads through underground rhizomes which will come right through the plantings on your property line. Here is a link to a Penn State publication on controlling knotweed. With its extensive underground root system, it will be very difficult to get rid of once it gets into your landscaping. It can be mowed, burned with a weed torch, hacked down with a sickle, but not defeated without chemical treatment because of its root system. When you dig it out, you may leave bits of root behind and those bits can regenerate the plant. Also, it appears to be able to change the soil chemistry so it is unfavorable to growing other plants where knotweed has been removed.

Thanks for your response, but unfortunately it is not helpful for my particular situation for a few reasons, the main two being that 2 of the 3 methods are not possible for me - chemical and mechanical control. The majority of the knotweed is not on my property, so I can't control that part. I can cut the part and weed the shoots that come up on my yard, as I do now, but not across my fence. Also, I will not use herbicide for many reasons, including that I can't afford it, there are other trees and plants in the area that I don't want to harm, I do not like to use chemicals or unnatural remedies, and that the herbicide would be mostly useless on only my property -- again because most of the knotweed is not on my property, but on public property next to mine on a big hill. So all that I think I can do is try to tame and control my side of the fence. I have called 311 about the city portion, but it is beyond my means and control to take care of an entire city hill of knotweed. Hence, I am looking for advice for my yard only, and not general knotweed advice. Can you make any specific plant suggestions for grasses or other plants to plant along the border of my fence that may make a fair fight against the knotweed as a form of 'cultural control' as your handout suggests. I believe the sooner I get some other plants in the area the better chance I have of finding a fair competitor for the knotweed. Thank you.

Unfortunately, J. knotweed is an invasive plant with no natural enemies. If it were me, I'd treat it like bamboo. Knowing that one main reason it is so persistent is an underground network of roots, I would dig a pretty deep trench along my property line and put in a rigid barrier to inhibit root penetration. I would keep my garden at least three feet away from the fence so I could monitor the advances of the plant, and scout my yard for seedlings. Remember that this plant will grow roots before it grows leaves and stems since it depends on the roots for propagation. I would dig it up carefully and bag or burn it. Regarding it like fighting poison ivy, I would look for first shoots and remove it quickly. Landscape cloth will do nothing to stop J. knotweed. Your best option is to keep a plant free zone between the fence and your garden so you can see any signs of it climbing the root barrier. You can kill top growth with vinegar, but the rhizomes are the thing to watch for. Also, since the plant seeds, it will blow into your yard or be carried there by animals. This is the best I can do to advise you. If you want more help, go to your local Penn State Extension and talk to the Master Gardeners. Here is a link to their location.