Japanese Knotweed

I recommend glyphosate (Round-up etc.). If you had big clumps, I would cut the stems, which are hollow, and pour a couple tablespoons of the stuff down them. If not, as seems likely in your case, spray the leaves. Glyphosate will kill any plants around, of course, so you'd need a different strategy if desirable plants were intermingled with it; painting individual leaves for example.

You might wait a couple of weeks to do it. Fall is the best time, because the roots are drawing down nutrients for the winter, and the poison gets them.

I have also dug up root systems and taken as much out as possible. I think poisoning now and doing this in the Spring would probably be best.

I mostly eradicated about 1600 sq. ft. of the stuff upstate in this way. If, however, the knotweed is also in your neighbor's yard, it will come right back to you, unless it's treated there as well.

David

Posted by: guest at September 4, 2007 3:35 PM

We moved into a new place full of the stuff and this spring I turned over the entire back yard. It actually worked pretty well - it has only held on in the spots under a falling-down wall that I couldn't reach. My mom recently sent me an article that appeared in our local paper (The Valley News, you can maybe google for it) that went through many approaches, and the only one they stood by was covering the area in black plastic for a season. I didn't find it that stubborn, but maybe I've been lucky. Some of the deep root I dug up were soft and stinky, so perhaps it was on its way out. Good luck!

Posted by: guest at September 4, 2007 5:00 PM

the more plants you plant in your garden, the less weeds you will have. Also, heavy layer of mulch is good for your soil and keeps weeds down.

Posted by: guest at September 4, 2007 5:59 PM

I am dubious about simply turning over the soil in an area as a way of dealing with knotweed. The risk is that small root fragments separate and become new plants. This is a major way in which the plant propagates. If the knotweed is well established, it will simply push up through black plastic; it comes up through my asphalt driveway. Several layers of plastic might keep small plants down, but, again, I'm skeptical. I don't think a single season is the test whether you've conquered this plant.

David

Posted by: guest at September 5, 2007 10:29 AM

Here's a thorough discussion of the whole problem. It concentrates on large-scale eradication, but you can get the picture about what works and what doesn't, and how to be careful.

David

http://tncweeds.ucdavis.edu/moredocs/polspp01.pdf

Posted by: guest at September 5, 2007 10:35 AM

Yes, I have the same problem. If any of your neighbors have it, there is really nothing you can do except constant maintainance since it will always come back. The roots are so deep and so strong, it's incurable.

I got selected for an HGTV show that was going to renovate my backyard and after seeing my Japanese Knotweed problem he told me it's not worth it to go through with the show. All of the work would be ruined in a couple months since it would still just come under my neighbor's fence. (I love in Ft. Greene.) The only solution is to hardscape it, basically cement it, and then make a decent backyard out of it after that.

Japanese knotweed can still come through cement if there is even a minor crack, so even this is not a permanant solution. But it's probably the easiest.

If it's just located to your yard, a year under a thick tarp is probably best, but still know that if any leave, stem, etc is around, that will turn into something, and it may still require a lot of maintainence.

It's just the biggest pain in the ass and I've tried everything to get rid of it (even calling HGTV.)

Posted by: guest at September 5, 2007 1:22 PM

It is not impossible to get rid of. I did it in a space about 40 x 40, but only on my property. A combination of manual removal (digging roots up carefully and thoroughly) and herbicides worked for me. I seriously doubt that covering or tilling alone will do any good, and the link I provide above tends to agree with this. Use herbicide now because it's the good time. Dig in the spring -- then when it reappears, dig or use herbicide again, or both. Year after next you may be OK. Really. Or maybe the next year.

David

Posted by: guest at September 5, 2007 3:51 PM

Been there done that.

We battled with our Japanese Knotweed for 3 years, first the organic route, pulling, turning over the soil (you would not friggin believe how vigorous and deep the roots are!), the black plastic for a year and a half, it was hellacious..

We finally gave in and consulted a nursery near the Terminal Market, he sold us (wink wink) the commercial grade Round Up, had to buy a special sprayer. MUST follow directions to a T and apply to the leaves of mature plants, no exceptions.

We now have a gorgeous garden (if not a disturbed water table!)
Good Luck.

Posted by: guest at September 5, 2007 9:27 PM

ps
in my experience it grows under black tarp and plastic
if it grows near the foundation of your home, it will offset it.

Kill it kill it kill it and have now mercy for it, it is an EVIL weed...there I feel better.

Posted by: guest at September 5, 2007 9:31 PM