Japanese Knotweed
The Japanese Knotweed is a quick-growing plant! It yearly comes up from the ground, and produces a thick-dense 10-12 foot tall screen. In mid-September, it produces a lovely display of white flowers, that attracts bees, and finally in October, when hit by a hard-killing frost, dies back to the ground. At that time, you take a really good pair of branch clippers, and cut it back to the ground, as CLOSE as possible. The Japanese Knotweed does not continue growing on the previous years growth, it ALWAYS dies back to the ground.
Over the years, the Japanese Knotweed snuck under the fence, and worked its way out to the street. It grew in the lawn, and even sent out roots under the poured cement foundation of the garage. Somehow, it resurfaced, through a crack, in the corner of the garage floor, where the foundation, and poured cement floor meet.
After a while, it was time to control, and hopefully eliminate the Japanese knotweed. Being that it was growing in clay soil, trying to dig it out would be too big a project, and it had migrated a distance. So my ONLY solution was to tame it with the lawn mower.
First of all, the Japanese knotweed does not give up, and die easily! So in early spring make sure any remnants of the previous year’s growth is cut as close to the ground as possible. You will be running your mower over that area, and do not want to hit any old stumps of the plant.
Starting with the first grass cutting, or when you see it starting to appear, WEEKLY cut it down with the lawn mower, close to the ground as possible. The Japanese knotweed is persistent, and will send up many consecutive shoots, trying to establish its ability to collect light (photosynthesis). From the beginning of spring, to almost late summer, it RELENTLESSLY tries to establish growth, since you are STARVING it of LIGHT. Only toward the end of “eradication season one”, will it start to slow down a little.
The following year, “eradication season two”, of the Japanese Knotweed, do exactly what you did in “eradication season one”. Cut it down weekly, don’t let any part of it establish growth, and STARVE it of sunlight. It will still try vigorously to send up shoots all season long.
Only in the middle, to end of “eradication season three”, after doing the steps of seasons one and two, will you start to see a marked change. The Japanese knotweed will start to slow down, the growth will be a lot less vigorous, and a lot weaker. I am on “eradication season three” inside the fence, and at about eradication season five, outside the fence, where it is almost non-existent. Hopefully in the next few years, I will have tortured, and killed off everything.
One more thing, a few times each season, I tried to get it with Roundup. It burnt the leaves, but really did not weaken the plant much.
So tell me, do you have the invasive Japanese knotweed on your property, and how have you tried to eliminate it?
Fred Gonsowski Garden Home 
Japanese knotweed (Fallopia japonica) is listed by the World Conservation Union as one of the 100 Most Invasive Plants. Its roots can destroy concrete foundations and blacktop roads. The United Kingdom is currently spending SIX BILLION POUNDS per year to control Japanese knotweed. There are at least four major banks in Scotland that will not write mortgages for property with Japanese knotweed on it.
DO NOT DIG! The primary way Japanese knotweed spreads is by root cutting. If you attempt to dig it up, you will only succeed in spreading the problem all over your property.
Second, stop mowing. Living cut stems that are left on the ground can root themselves. This is a great way to increase your problem. If you must cut live stems, make sure to bag them carefully and either burn them or dispose of them in sanitary landfill. DO NOT ATTEMPT TO COMPOST!
Japanese knotweed has both male and female plants. Only the female plants produce seed, however, they both procude flowers, which look almost identical. If you are not seeing seeds, you may only have male plants. That would be a very good thing. While Japanese knotweed seed viability is very low, it produces a great amount of seed, so if even a few of those seeds are viable, they can still be spread by wind or water.
Since you have already started mowing, you will no longer see the nice big hollow bamboo like canes. That is unfortunate from the standpoint that there is an injection tool that can be used to fill those hollow canes with a highly concentrated form of RoundUp. That is an excellent way to get enough herbicide down to the roots in order to reallys set it back that first year.
Best approach for you is to let the stems and leaves grow next season until the get about two feet high (In WI that is in late June of July). At that point they will have enough leaves to take up a good amount of RoundUp Mix it at the prescribed rate for foliar spary and spray the entire leaf surface of all plants to the point of run-off. Yes, one application of RoundUp will not kill the roots, but it will weaken them. Repeat the spraying a second time (here in WI that would be in September).
It will take a minimum of three years of this regimine to exhaust the entire clone. The more likely scenario is up to five years. Monitor for two years after you spray the last shoots to make certain it is completely dead.
Hi Mark Horn! Thank you for your comment. I love a LONG one. I am all for different opinions on how to handle a subject. Your way of doing it, might be different from mine, BUT different ways give the readers options to consider, when tackling their project. Again thank you! As I take this blog further along (adding pictures, etc.) I hope to hear from you again. Happy Holidays.
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Did you find that mowing it cause any spread of the knotweed? I have read that, although the roots seem to be the most easily spread, fragments of the stem and canopy are also able to produce roots and cause new infestations, which is why here they are now posting signs warning of invasive plant species warning maintenance crews not to cut sections with the tractor mounted flail cutters as this would produce potentially millions of new plants. I have read a few local articles that have said herbicides are the only sure way to get rid of it, but after reading a little about glyphosate, I would rather avoid it.
Hi there Nick. After mowing the area, you could rake up the space, so no plant material is left. I got rid of it at my mothers, and never saw cut branches re-rooting. My advice is to cut it down to the ground (after it is done flowering) and bundle up the branches and throw it all away. Next Spring when you start seeing it growing, be diligent about cutting it down EVERY WEEK, no matter if you are in the mood to do it or not. The plant will fight YOU tooth and nail to survive. If you give it an inch, it will try to win out over you!. Good Luck and Fight the plant to its DEATH
I have only just realized that we have had Japanese knotweed growing just over our garden fence in the hedgerow of a local farmers field for the past few years, I noticed during the summers months small shoots appearing thro our patio, after reading up on this plant I am quite concerned about the damage this plant can cause, and am unsure of what to do next!! Do I contact the local farmer and tell him my concerns??
Hi there Charm Knight, sad to say, there is no law that says the farmer can’t grow the knotweed on his land. You could ask if it would be all right if you tried digging the plants out that grow along your land, but that is not the easiest thing to do. Plants like that are really destructive, but a lot of people are just too lazy to put the effort into eradicating it. Thanks for your comment.
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Does anyone know if black sooty mould kills Japanese knotweed? Or retards its growth?
(Mould is UK spelling: US spelling “mold”)?
Hi there Rita did you read this article, I found on the domain? It hopefully will answer your question…Good luck with your Mould in the UK http://baker.ifas.ufl.edu/Horticulture/documents/BlackSootyMoldonLandscapePlants.pdf
Hi Fred,
great informative article and well written 🙂
I’m making a website for a friend (on a very low budget!) with a small business removing invasive weeds in Ireland. I wondered if I might be able to use your lovely Japanese knotweed illustration on one of the website pages – I would of course link it back to your post and give you full credit for the image, or however you would prefer.
Thanks,
Sally.
Hi there Sally, yes you can use the illustration, as long as you link it back to my site. Good luck with your project.
Thanks a million Fred, much appreciated 🙂
I see your original article was in 2010. What is the state of the knotweed today? Is it eliminated? Did you have to resort to herbicide treatment.
Hi there Ray, I’ve eliminated 99.9% of the knot weed, but if there is even a shred of the root left in the ground it will regenerate into a plant. I redid the area where the knot weed grew in my mother’s yard, and turned over the dirt and eliminated any signs of old roots. BUT, I do see, here and there, a plant that tries to come back, which I hit with roundup and kill it. If you break off the top of the plant, and don’t kill it to the root, like a carrot it can grow back.
Hi Fred,
I love your art! I am mobilizing a group of enthusiastic landowners in my area to start a community project to eradicate it along the river. I am wondering if it would be alright for us to use your art (with credit given!!) in some of our posters and flyers to help get people involved? If we use it online we will also link back to your site.
Thanks so much,
Isobel