Brown Knapweed

Asked August 4, 2020, 3:51 PM EDT

I have brown knapweed that has taken over a good portion of the sides of my driveway and is spreading everywhere. What is the best method of killing it and can you treat it now with any adverse effects to other plants or animals? Any help with this would be greatly appreciated

Pine County Minnesota

5 Responses

Thanks for the question.

You may know that Brown Knapweed is considered by Minnesota to be an invasive plant. Probably not telling you anything that you’ve not already discovered. You will need to use chemical herbicides. Trying to dig things out may not be effective due to its extensive root system. Moreover, trying to dig it out along your driveway may not be practical. Knapweed shows its most vigorous growth in the spring and this is when you should apply herbicides. I would not suggest the use of glyphosate due to the presumed proximity of your yard.

Readily available selective herbicides effective in controlling knapweed contain 2,4‐D, a combination of dicamba and 2,4‐D, or a combination of triclopyr and 2,4‐D. If using 2,4‐D or triclopyr, re‐treatment will probably be necessary to control things.

The following should give you some additional information:

Good Luck!!

Thank you very much, Steve! Would Ortho Weed B Gone with 24D and MCPP work for the knapweed? Is it safe or effective at all to try to kill it right now? Or just in the spring? I just do not want to let it seed out or whatever as fall approaches and not be able to control it this spring. Thank you again.

Thanks for the response.

The Weed-B-Gone should be effective. A herbicide like this is most efficient when a plant is actively growing. That is the situation in the spring and that is why it is the preferred time of application. Certainly no harm is done if you also applied it now. You are correct that it would be good to kill this weed before it spreads its seeds. If flowers are still present on the knapweed, that means that seeds have not yet been dispersed and for these plants, a herbicide treatment could be done. How effective it will be is uncertain but certainly there’s no harm in trying. I would suggest that you consider subsequent applications at the end of this month and at the beginning of September.

I suspect that this plant may still visit you in spring 2021. It is very difficult to kill. So at the first indications of its appearance next season, again apply this herbicide.

With persistence and patience, you will be able to get rid of it.

Good Luck!!

Thank you for such a quick response time!! I am going to take your advice and treat it several times this late summer/fall and attack it again this spring as it starts. Thank you for all your help!

Glad to have helped.

Thanks for using this forum.