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I have a Japanese Knotweed problem in a limited area (about 15x15 feet).

It is very difficult to dig up the roots because they can be a good 2 feet underground.

Therefore I am considering death by injection: cutting off the stalks about 6 inches above the ground and pouring imazapyr into the well of the stalk. Will this be effective? Is there a better approach?

One pamphlet I read described an experiment where they did something similar but used a mixture of glyphosate, triclopyr, and imazapyr. I am not really expert in different herbicides, so I am looking for some expert advice here.

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The usual method is to pour glyphosate down the hollow stalks,not sure how well the one you mention will do. Repeat and ongoing treatments are necessary,but one other thing - if you're going to cut down any of this plant, don't compost it, don't carry it anywhere without bagging it up first, and if you can, burn it, if not, where you are, it should be possible to put it out with the trash. In fact, its probably safest to put each stalk you cut off straight into a bag rather than laying it on the ground till you have a collection, then bagging it. The tiniest fragment of this plant inadvertently dropped on the ground will grow. And, by the way, the roots actually go down 9 feet... information here re various treatments tried in your area,along with a note at the bottom about being able to bag it up and put it out with the trash

https://www.cctvcambridge.org/Japanese_knotweed_Harvard_MIT

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All photosynthetic plants need light to make their food. If light or photosynthesis is denied, either by continually cutting off new growth or burying very deeply and combining the cutting of any green growth any plant will die. This exotic 'weed' is one of the top most prodigious of all, only surpassed by purple loose strife and that European Birch, Paper?! All those chemicals would only make everything worse. Would not help at all. I am a licensed commercial pesticide operator, supervisor and I know that it is far easier to not use pesticide and learn how to garden correctly as to prevent having to use bandaids for unnecessary mistakes. I allowed our electric company who were in the process clearing electrical lines to dump their chips over this huge area of blackberries 6' deep. Caused this cool fog all winter long from the heat and by the spring 6 feet ended up 6" deep and not a single blackberry ever came back. For Equisetum we routinely weed wacked the green growth and after 2 years the Horse Tail was gone. I've tried chemicals from time to time but I am serious, they aren't worth the trouble.

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    Numerous white papers published by different states and US forestry service say that herbicides are the only effective measure against Japanese knotweed. I have been using mechanical methods against the stand I have for 16 years with no success. Frankly I am tired of digging 5-foot deep holes trying to get out roots that keep going deeper and deeper. – Andre Lenotre Jul 26 '16 at 8:40
  • I'd go get some cheap construction debris chips (shoot there is a name for this stuff I've just had a senior moment) and dump it right on those plants 4-6' deep, without cutting the plants down. No photosynthetic plant can grow without light. These guys have food stored in their roots but you can starve them by burying them in this mulch and keeping them buried by adding more mulch the next year if necessary. Easiest way to kill noxious plants. Do it before they flower and go to seed!! Keep a spray bottle of Round up to hit baby plants on perimeter. – stormy Jul 26 '16 at 20:17
  • My goodness, am I losing points because I am not advocating chemicals? If chemicals worked I would understand as Knotweed is an incredible survivor and exotic out competing indigenous plants. How funny. US forestry service must know what they are talking about, huh? They do not deal with noxious weeds but the Cooperative Extension Services do have experience and testing behind their recommendations. I would like to know why my comments are wrong. Am I not addressing your problem? Are you wanting only chemicals to use to kill knotweed? If I had seen any evidence chemicals worked... – stormy Jul 26 '16 at 23:43
  • Your way would surely work but how long would it take to deplete the roots of their carbohydrate storage? – Graham Chiu Jul 28 '16 at 7:28
  • @stormy: cutting and burring is not so effective: with cutting you risk to disseminate the plant. Burring: it seems it needs 10 years of plastic, and not success guarantee. 15x15 feet requires a lot of effort. Chemicals works (in Europe) because we have only one female clone (so only one individual), no risk of genetic selection. – Giacomo Catenazzi Dec 13 '16 at 12:44

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