I absolutely hate Japanese knotweed with every inch of my body! It grows anywhere and everywhere on the South Shore of Massachusetts. My question for you wonderful people out there is, “What is the best way to effectively kill and maintain areas where Japanese Knot Weed grows.

Some attempts I have made to destroy this “super weed” are cutting it with a brush cutter attachment on a straight shaft multi tool weed whacker. After it is cut as low as it can go I spray the stocks that are left with a generic round up from Home Depot/Lowe’s. After it is cut and sprayed I covered it with a large black tarp (during middle of summer) so it would get baked in the sun. After 3-4 months new growth was shooting up in same places.

Another attempt I made started by cutting the full grown Japanese Knot Weed down to about 3 ft stocks. Then we pulled the stocks out and shoveled the roots as best we could. Then the black tarp for about 2 weeks. This kept the area of Knot Weed gone for about 5-6 months before new growth happened. But then it was late Fall heading into a north eastern winter. In the spring the Knot Weed came back but it was much smaller than the previous year. I sprayed the new growth every 2 weeks throughout the spring/summer and that worked well.

The best method I was able to do for a customer of mine who had a massive patch of Japanese Knot Weed and saplings growing was to use my skid steer and scrape down about 3-4 inches and remove the entire root system of the organism. The Japanese Knot Weed has very shallow roots so 3 inches of scraping gets nearly 100% of root system. I then brought in about 10 yards of nice screened loam and seeded the entire area. This has lasted from early May 2018 until now, mid July 2018 and we have not seen on shoot of Knot Weed.”!!

My question for all you wonderful folks is if anyone has a specific chemical that can be sprayed. The generic round up works... kinda. But there must be a stronger or more target product that is designed for this invasive species!

The Japanese Knot Weed is my all time, most despised enemy in the world of invasive species and large property maintainence! If anyone can relate and any tips, solutions, or just agreement on the hatred of this organism feel free to comment!!


  • Will add pictures shortly! Jul 19, 2018 at 7:42
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    The rhizome root system on Fallopia goes down nine feet - there is a smaller version, 'compacta, which doesn't go quite so deep. The biggest issue is how you deal with what you remove - in the UK, specialist companies wear hazmat suits and special precautions are taken to ensure not a single fragment is dropped anywhere. This information is for the UK, but may be of some interest rhs.org.uk/advice/profile?pid=218 Usual home control is to cut down and drip stumpkiller into the hollow canes. Here,, only specialist companies have accees to stronger weedkilling products.
    – Bamboo
    Jul 19, 2018 at 10:24
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    Nine feet? This is fabulous stuff on a plant I THOUGHT I knew well! I should have charged a ton more moola! I just don't see the stump killer being effective for a herbaceous plant but they do say to inject glyphosate into the canes. I have to go back and review how glyphosate works because that makes no sense to me. Without vigorous growth and being translocated as the carbs are taken down to the roots I am not seeing how glyphosate would work. Love the hazmat suits!!?? Really? Glyphosate works well, anything 'stronger' has shown to not be effective.
    – stormy
    Jul 20, 2018 at 22:09
  • @Caseyslandscaping Your question has 'juiced' us up. Me at least! I've spent the last few HOURS reading up on this plant. Added a few notes to my answer and will most likely be back when I learn more. Giacomo's comment about non viable seeds has me hooked. Know thy enemy is oh so true!
    – stormy
    Jul 20, 2018 at 22:12
  • @Bamboo Seriously, it is classed under Social misconduct and crimes? Wow. Are other 'noxious' weeds classed here as well? What an incredible plant you just gotta admit. I am still looking to find why most Fallopia are sterile females. Nine feet? Your article said that burning was a good idea after allowing glyphosate to kill the roots. I would spray some oil then flame ON! I think Casey might be doing some major pyromaniac stuff soon! Grins!
    – stormy
    Jul 20, 2018 at 22:23

1 Answer 1


The best way is to starve them out. Cut them down before they flower. Keep taking the top growth off the plant and the plant is unable to make its own food via photosynthesis.

Another method is to bury them if there is room. No plant can withstand no air, no water, no sun.

Depending on the conditions, the environment, the commercial services spray lightly with glyphosate during the late spring, a month before flowering and while the plant is vigorously growing. Wait an entire month and then cut the entire patch to the ground. Cover with clear plastic, not black...and staple the edges securely to the ground. I would also bury the edges with shovel fulls of soil.

Note: Rake up all debris and bag carefully! Every little tiny bit has to be removed because this plant can grow from just a wee little bitty tiny piece! I'd get a shop vac! Using a blade weed wacker instead of a line trimmer would also be a good idea. The line trimmer would mangle and chop too much. The blade would cut the canes down more cleanly. Use a big heavy tarp and carefully gather the canes as you work laying them down on the tarp so you don't walk all over them and break them up. I've been doing some research on this since I answered your question the first time and WOW. I think I am understanding much more clearly your frustration! I've not had the problems I could have had, I was lucky my clients had time to wait.

Hopefully there is plenty of summer left to 'sterilize' the soil and remnants of this plant.

Towards the end of fall, remove that plastic (please!!) and dump 1 to 2 to 3 feet of topsoil over all for the winter. There is lots of other stuff to dump on top for the winter but hey, no way can a plant survive this...but THEY do!

Follow up with hula hoeing or spreading a bit of glyphosate on any new growth in the spring.

I know this 'weed' well. I am from the Pacific Northwest. I've been a licensed pesticide applicator for decades and there is no herbicide that is able to get this under control within one year like this more manual management. Think of it as job security. Educate your clients about this very noxious exotic weed. Break your services down into manageable packages.

This knot weed is not your enemy. It is just a smart plant. Your Cooperative Extension Service in...Massachuchetts (sp) grins...is your best friend! Get to know them. Do you have access to human poo/sawdust decomposed mulch? This would be your favorite rich uncle.

What you've been doing is very close to the best you can do. Spray only when the plant is in vigorous, early season growth (glyphosate). Do not spray so much you get drip and certainly not during any stiff breeze. Allow the glyphosate to kill the roots; at least 3 weeks.

Then cut the plants down to nothing. Make sure you get every little piece of debris picked up.

Solarization is done with clear plastic, not black.

Remove the plastic during the fall and bury the best you are able. I used the Power Company's chips to get rid of blackberries, almost as bad as Knot Weed. 6' piles, 10' wide. Added alfalfa pellets for nitrogen. Steamed all winter long. Kind of cool.

In the spring those 6' piles were 6 ", seriously, and I never saw another blackberry in that area.

Don't forget weeds are just plants. Weeds somehow have gotten this reputation and becoming everyone's top monster in the garden. I don't get it. Weeds just are not at all a problem. Regular maintenance, a little at a time, starving them out, smothering weeds and/or using a mulch without any weed seeds...

Note: I've just learned that the flowers don't produce viable seed, so you can stop worrying about the timing, this plant is simply amazing at vegetative reproduction, however. Don't mess with patches of this plant along waterways. You have to alert the Weed Cops (Cooperative Extension Service) and they are licensed for pesticides and water. The only way to eradicate (?) this plant is a multi weapons approach and glyphosate is important.

Glyphosate is safe to use if you follow the directions. More is not better. Perhaps multiple applications but not spraying every inch. There is one mother root system generally. Whatever part of the plant you spray is translocated to that mother. Killing the roots by interrupting some major systems. Quite brilliant. It is not a horrible toxic chemical. It will not stay in the soil. You most certainly can grow vegetables in that soil some day in the future.

This plant was brought to the states in 1850? Something like that and it is now in every state and in Canada and in Mexico and is a huge problem. This is one plant that warrants using a Pesticide. Hands down. Forget vinegar. Forget hot water. I wouldn't go so far as injecting glyphosate into the canes. I am still researching that bit. Makes no sense because the translocation process is how glyphosate works.

I just had an idea about the debris. A ghost buster type flame torch! After allowing glyphosate to do its job, I wonder if torching SMALL patches would be effective? I think you'd LOVE that method, huh? Grins!

There is also some Patent Pending! method? Chemical? to eradicate Japanese Knotweed! That should make you happy as well. It is called 4 Stage Model. I've got to check out what that is all about!

the Best way to control Japanese...

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    Why "before they flower"? Do you know that (apart of Japan and Asia) it is always just one unique female clone. Just one individual, no seeds. Jul 20, 2018 at 10:10
  • No, I did not know that, Giacomo! Really. So they flower and don't set seed?
    – stormy
    Jul 20, 2018 at 20:33
  • And this is NORMAL? for this plant?
    – stormy
    Jul 20, 2018 at 20:33
  • @GiacomoCatenazzi I haven't been able to find WHY they don't have viable seed...yet. Boy howdy have I learned a lot more about this plant! I'm adding more to my answer. I am also wanting to know more about the timing of glyphosate for this plant. They say that the translocation works best at the end of the summer, not the beginning? Do you know glyphosate well? Why would they INJECT glyphosate into the canes? I am not seeing how that would help at all. Do you have a link you could send me on the reproductive powers of Fallopian japonica japonica?
    – stormy
    Jul 20, 2018 at 21:25
  • I do not know. Often just covering with plastic for many years (which is IMHO also bad). I do not know why glyphosate do no work. In Japan it reproduce normally, but nobody is crazy now to import such plant, so the actual plant we have is just a cut from a real plant. This plant growth easily from a small piece, and it likes to be ferried with cars and rails and water. It is invasive for this reason. By walking near such plant, one risks to bring it in new places. All vegetative reproduction. [Note: bamboo instead will flower, but after 100 years and then it dies] Jul 21, 2018 at 10:04

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