Japanese Knotweed HErbicide Application - Ask Extension
Hello there,
We have Japanese knotweed growing up from our neighbors yard into ours. There is no water near and the patch seems to be limited to o...
Knowledgebase
Japanese Knotweed HErbicide Application #908258
Asked July 02, 2025, 7:12 PM EDT
Hello there,
We have Japanese knotweed growing up from our neighbors yard into ours. There is no water near and the patch seems to be limited to our yard and hers (Grrrr.) We know what it can do and how bad it is, so we are looking to be as proactive as we can be now that we understand what we are dealing with. We cut back knotweed in early June according to many instructions that we’ve read. Our hope is to get the stems injected with glyphosate in late summer/early fall (two weeks before the first frost).
Do you have a recommendation for which herbicide or combinations of herbicides we can inject/apply?
Help!
Multnomah County Oregon
Expert Response
Emily: You are correct in knowing that the Knotweed group (Japanese and others) are a pain to control. Your procedure of cutting them back now and then treating in early to mid-October is also very good. That technique has usually been as successful as one treatment can be with these plants.
Glyphosate is well-absorbed through the leaves. Cutting and painting glyphosate on the cut surface is incredibly time consuming and, in one location, was applied at a very high rate using way more glyphosate than the normal spray process of glyphosate, with no better results.
Glyphosate is also gentle on landscapes where tree roots may be below the area where you are spraying the mix. Glyphosate is tied up the minute it hits soil so it can't unintentionally hurt roots where the knotweed is unless you get the spray on leaves of a tree.
All that being said, the first application will knock back 95% of the know weed. But this is where it gets complicated. Next year's knotweed growth will look contorted and just plain weird. You can cut them but don't spray them. Wait until the following year to spray. This formula almost sounds like voodoo but it has come from lots of experience in the PNW on the knotweed group.
There is another active ingredient, imazapyr, that is paired with glyphosate in a homeowner mixture. Imazapyr is better than glyphosate but has the root issues mentioned above. It is the main chemical used on knotweed where it won't damage woody plants. If that weren't an issue (roots can travel a long way from the trunk of a tree or shrub), it might be worth a try. Timing would be the same or possibly a little earlier to mod-October. Hope this helps. Chip Bubl, OSU Extension Agent retired, Columbia County. <personal data hidden>
Glyphosate is well-absorbed through the leaves. Cutting and painting glyphosate on the cut surface is incredibly time consuming and, in one location, was applied at a very high rate using way more glyphosate than the normal spray process of glyphosate, with no better results.
Glyphosate is also gentle on landscapes where tree roots may be below the area where you are spraying the mix. Glyphosate is tied up the minute it hits soil so it can't unintentionally hurt roots where the knotweed is unless you get the spray on leaves of a tree.
All that being said, the first application will knock back 95% of the know weed. But this is where it gets complicated. Next year's knotweed growth will look contorted and just plain weird. You can cut them but don't spray them. Wait until the following year to spray. This formula almost sounds like voodoo but it has come from lots of experience in the PNW on the knotweed group.
There is another active ingredient, imazapyr, that is paired with glyphosate in a homeowner mixture. Imazapyr is better than glyphosate but has the root issues mentioned above. It is the main chemical used on knotweed where it won't damage woody plants. If that weren't an issue (roots can travel a long way from the trunk of a tree or shrub), it might be worth a try. Timing would be the same or possibly a little earlier to mod-October. Hope this helps. Chip Bubl, OSU Extension Agent retired, Columbia County. <personal data hidden>
