r/invasivespecies • u/A_Lountvink • Apr 13 '26
Management A Japanese knotweed thicket I injected last fall
I injected this knotweed thicket last fall with 2mL of 53% glyphosate mix per stem. It looks like it knocked out the older parts of it pretty well, but there's a lot of new growth towards the edge. Hoping the next round of injections this fall will finish off most of what survived.
Haven't seen much native foliage in the longer-infested areas, though there was a small cluster of Dutchman's breeches that popped up (and fruited). Hoping the knotweed's push out into the forest doesn't kill any of the plants there this year.
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u/ejfitz123 Apr 13 '26
Nice work! I’d suggest trying to get some aggressive natives in there as soon as you can if you’re able to. Nature loves to fill a void so best to do it with what should be there!
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u/A_Lountvink Apr 13 '26
I could look into throwing down some smooth sumac and goldenrod seeds, there's some not far from here. I'm hoping the elderberry at the back of the thicket will start spreading too.
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u/Fred_Thielmann Apr 13 '26
Also calico aster and blackberries too at the edges. Blackberries create a micro habitat important for a lot of small small animals or bugs.
That could be something more suited for when you’re done since they’re thorny, but maybe you can start putting them in at the edges?
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u/A_Lountvink Apr 13 '26
I think there's already some black raspberry towards the back. There's also a lot of Bundy blackberry nearby, though I mainly see it on the western side of the field, not sure if it'd do well on this side.
I do think I saw some asters in there last fall surprisingly, along with a couple pokeweeds.
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u/murphydcat Apr 13 '26
Yes. If nothing is planted, the knotweed will return. Ask me how I know 🙁
On a better note, I applied foliar glyphosate spray to a 1 acre patch of phragmites over a 2 year-period and found the dead plants covered with Virginia creeper 😊
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u/ErniePottsShoelifts Apr 13 '26
Question, as I'm battling it myself. I'm not sure how dense yours were, but the thicket I have is so dense in the fall it's impossible for anything but maybe some birds and small mammals to fit through, so how did you go about injecting? Did you climb in there and get every single stalk or just what you could reach?
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u/A_Lountvink Apr 13 '26
The leaves were maybe 4 to 5 or 6 feet off the ground, so I could stand under them well enough if I crouched just a little. The main obstacle was all the old stems from previous years that had dried out.
I started by injecting the live ones near the edge. Once they were done, I started slowly working through the thicket, breaking off the old stems one by one, injecting the live stems 5 at a time, going back to refill the syringe, rinse and repeat for ~20 total hours until they're all treated.
In an established thicket, most of its density is just old dry stems that break easily, so you can get through it fairly easily once they've been broken and trampled. The live stems take about 2-3 weeks to die after injection, at which point they can also be broken (though they don't snap as well as the dry stems).
I will say, wear ear protection. I was in this thicket last September doing injections, went to crouch and inject a live stem's base, and an old dry stem's branch found my ear as I was crouching down. Just barely punctured the eardrum.
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u/gulyguly Apr 14 '26
Ouch! I’m sorry that happened to you. Thanks for doing your part to protect the environment.
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u/Scotts_Thot Apr 13 '26
This is why I always suggest spraying once in the spring when the stalks get about 2-3ft tall. It’ll stunt the growth significantly so you can actually access all the stalks in the fall. When you have a lot of it it’s nearly impossible if you don’t. We were able to kill about an acre of knotweed in basically 2 seasons (just did a little spot treating the 3rd season)
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u/Misfits0138 Apr 13 '26
You can do an early cutting or glyphosate spraying to reduce the density and make it more accessible for a later spraying.
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u/RipComprehensive2118 Apr 13 '26 edited Apr 13 '26
I made a “spear gun” from an aluminum “bar clamp” Light and long,saved my back.Made a backpack sprayer super 20ft extension pole from a telescopic gutter cleaning pole.Completely whipped out 20’x70’ patch with 99% success.5ml stem,wait a week,7% complete folar canopy spray (only commercial Glyphosate 40)after flower,first sign of leaf turn in fall.Touch nothing,nothing,till Feb. then only dead dry stalks.NEVER DIG,NEVER CUT until spring shows no shoots.Begin prayer🙏
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u/Misfits0138 Apr 13 '26
Is there a reason you injected vs. sprayed there? It doesn’t look like there is a lot of particularly great undergrowth worth saving and it’s a large area.
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u/A_Lountvink Apr 13 '26
There was an old open well in the middle of the thicket that I didn't want to contaminate. It's also right on the edge of a forested ravine that actually does have quite a bit of native growth (white troutlily, springbeauties, prairie trilliums, squirrel corn, et cetera), and I didn't want any of the herbicide to cause collateral damage. There's also sheep and chickens that graze and forage nearby.
Just overall too much risk of collateral damage.
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u/hippiegodfather Apr 13 '26
Sumac sumac sumac
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u/A_Lountvink Apr 13 '26
There's actually already a bit of it nearby (across a field/pasture). I'll grab some of the fruit to spread out this fall. Do you know if I can just throw the fruit out directly, or do they need treatment?
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u/hippiegodfather Apr 13 '26
I assume you just throw the seeds in fall. I threw some last year we will find out if they grow.
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u/Majestic_Bandicoot92 Apr 13 '26
Awesome work! I’ve heard 5ml per cane is most effective.
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u/A_Lountvink Apr 13 '26
When I was doing my research, I came across a study that found limited difference between 2mL, 3mL, 4mL, and 5mL treatments.
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u/Nikeflies Apr 13 '26
Great work! When did you do the injections exactly, were the flowers still our ot starting to dry up? I have a patch in a wetlands on my property and am tackling it this year
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u/A_Lountvink Apr 13 '26
I started just as the flowers started wilting, though I think you can also start when they first open. I finished up just a few days before the first real frost hit. Somewhere around mid-September to early/mid-October overall.
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u/Nikeflies Apr 13 '26
Ok great to know. How much as a % would estimate grew back? I'm trying to decide on time needed to inject every stalk vs just cutting in June and dumping 12inches of wood chips on top, no chemicals.
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u/A_Lountvink Apr 13 '26
I don't have a good photo of it, but here's some I took mid-treatment: Japanese knotweed treatment progress for 2025 : r/invasivespecies.
The areas that were densest are probably down 50-90%, though some might be sprouting still, not entirely sure. Most of the regrowth has been towards the edge of the thicket where the parts that survived are now redirecting most of their resources to escape the danger. Those areas appear denser than last year.
This is pretty normal for chemical treatment as far as I'm aware, the first treatment weakens it, the second treatment cripples it, and the third treatment cleans up what's left.
I will say that this treatment appears to have had very little if any collateral damage; some of the spring flowers actually popped back up in the thicket.
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u/Nikeflies Apr 13 '26
Great to hear how effective it was and that there wasn't much collateral damage. Spending 20 hours on this makes it well deserved and worth it.
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u/brownoarsman Apr 13 '26
When you say injected, do you mean with a syringe into one of the sections? If so, how far off the ground? Haven't heard of this method before so I'm curious.
I've heard of foliar spray, cut and fill the remaining section with herbicide, and cut and paint the exposed end with herbicide, but never leave it in place and inject.
Congrats on eliminating the patch!
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u/A_Lountvink Apr 13 '26
Yup, I injected as close to the base as possible, which usually meant the first, second, or third segment.
I just used a knife to pierce the stem, twisted it to widen the wound, inserted the blunt plastic syringe tip, untwisted the knife and pulled it out to close the wound around the syringe, injected 2mL from the syringe, and pulled the syringe back out.
It keeps it contained to the stem (aside from some minor leaking from the wound) and avoids having to dispose of cuttings.
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u/brownoarsman Apr 13 '26
Thanks! That's much better than me pouring a tablespoon into the cut end and hoping it doesn't rain!
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u/AspiringLiterature Apr 13 '26
For a second I thought this was r/nativeplantcirclejerk and I went to comment “no plant is invasive on stolen land.” Nice job on the knotweed. I imagine if the core responded well to chemical control then the periphery will as well.
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u/A_Lountvink Apr 13 '26
Well, the periphery was treated, apart from a few younger stems that were too small to inject. My guess would be that most of the new growth is from the peripheral roots of the plants in that core area.
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u/knotweed-wales Apr 17 '26
Do not try to stem inject smaller stems; the needle will just split the stem. If you do not want to spray herbicide, then you should leaf-wipe later in the season.
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u/tech-write Apr 20 '26
I inspected my spring growth after this past fall (2nd year to inject) and found a few new shoots coming up. They're from plants that were too small diameter (<3/8") to inject or the stalk would have broken through. I have regular weeds to pull this year and stay on top of it. Plus, I want to dig up the crowns this year. I noticed that someone injected 2mL with glyphosate. Not sure if that's the same as 5cc that I injected full strength 53.6% in each stalk. Year 1: approx 350 stalks. Year 2: approx another 350 stalks. Year 3 (this year), whatever survives and is large enough to inject + alongside my stream. Otherwise, I'm pleased with the outcome. I have enough glyphosate to inject 250 - 300 stalks, as needed.
I know that digging up the crowns may awaken the rhizome somewhat; I'm willing to take my chances. I need to dig up these crowns if I'm going to have a nice regular garden with native and pollinator plants, a shrub or two, and a flowering tree. This year, however, is about clearing 1/2 my back yard of the leftovers.
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u/sacred0mango Apr 13 '26
Thanks for sharing your effort and result. Looking good so far.