A large portion of my yard is infested with (non-native) Japanese knotweed, which is nearly impossible to remove without herbicides.

I have been waiting until late summer/early autumn to begin an eradication campaign using glyphosate. In general I am very careful with how I use glyphosate/Roundup, only using it when dry weather is predicted and only in cases where manual removal is impossible.

I noticed a few Monarchs flying around my yard today, and I am worried that this could kill them if they light upon a treated plant. Most of the literature I was able to find points to an indirect impact of commercial-scale eradication of milkweed, which is the sole food source for larvae. However I do not plan on spraying milkweed (if I even have any in the yard) and I cannot find much about the direct impact of glyphosate on Monarchs or other butterflies.

Is this something I should be worried about? Ironically, I am planning to plant a butterfly and pollinator garden once the knotweed has been eradicated. If I wait a few more weeks for the Monarchs to migrate south, will it be too late for the glyphosate to be effective?

2 Answers 2


I have a hard time with injecting glyphosate into hollow stems for treatment. I've read this as well but the way glyphosate works is to be taken up by the leaves, vigorously growing leaves and it is incorporated into the physiology of the plant's chemical processes to mess with this 'enzyme' without which the plant dies. Taken into the plant this way takes the glyphosate down to the roots. Cutting down the Knotweed should be done 3 weeks after spraying.

Glyphosate: mechanism of action. Glyphosate is a herbicide used in agriculture and non-crop situations for the control of a wide range of weeds. ... Once absorbed by the plant, glyphosate binds to and blocks the activity of the enzyme enolpyruvylshikimate-3-phosphate synthase (EPSPS).Jun 19, 2013 Glyphosate | Glyphosate: mechanism of action https://www.glyphosate.eu/glyphosate-mechanism-action

To fix your problem, spray at night using large drops not a fine spray. These plants are usually ONE mother plant. Literally. Just spray some of the leaves here and there. Glyphosate will be taken up and transferred throughout the plant including the roots. Chopping off the bulk of the photosynthesizing plant factories doesn't make sense at all. Going through the photosynthesis process is how glyphosate works to become systemic. It doesn't work at all by contact like hot water, vinegar and acids. I have read other articles that tout this method. I would spray at night using a sprayer you can control the droplet size. It will dry before the butterflies and bees become active. Wait 3 weeks then cut them down and inject with a big fat syringe into the canes.

Be careful cutting the plant down. Don't use a line trimmer/weed wacker. If you can, use loppers with long handles. Keep a tarp next to you to put the debris on as you gather the canes and foliage, raking up the dead stuff already on the ground carefully. Any part of the plant has the ability to regrow.

You'll be doing this once a year for a couple of years. You are essentially starving the Mother plant as well as killing the roots with glyphosate. Disturb the soil as little as possible. Pulling out roots leaves too much chance of breaking the roots starting a new 'Mother'.

Consider dumping soil or compost or bark chips on top of the soil about a foot or more deep.

  • "You'll be doing this once a year for a couple of years" - Does this mean that any areas infested with knotweed are essentially unplantable for several years?
    – alexw
    Commented Aug 22, 2018 at 22:45
  • Yes. If you want to get rid of this major gnarly plant, yes. Sorry to be the bearer of bad news. We have a bunch of other question/answers concerning Japanese Knotweed, have you checked them out? This will give you a better idea of what you are battling!
    – stormy
    Commented Aug 22, 2018 at 22:50
  • What are your plans, Alex? A picture or two of your landscape, an idea of what you are hoping to do will help tremendously. Sometimes these 'sentences' promote the best ideas ever.
    – stormy
    Commented Aug 22, 2018 at 22:53
  • it's a little off-topic for this question, but maybe we can talk about it in chat?
    – alexw
    Commented Aug 22, 2018 at 23:21

You're right in that the effect on Monarch butterflies is indirect - its caused by glyphosate being used on milkweed. But if you are going to treat Japanese knotweed with glyphosate, you don't need to spray it - it's more effective to cut off the tops of the plants to reveal the hollow stems, and drip glyphosate down into the stem, rather than spraying the whole plant. It's more time consuming, obviously, but is the method of choice for amateur attempts at dealing with the plant; this method also has the environmental advantage of not affecting flying insects such as butterflies when compared to spraying. Some information on that in the link, though its aimed at the UK market - scroll down to chemical controls https://www.rhs.org.uk/advice/profile?pid=218

Note that it will likely take 3 or 4 growing seasons (effectively 3-4 years) to eradicate the plant, and that all and any regrowth must be re-treated.

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