Spotted Knapweed

August 1, 2023 by Liz Wiese

If you’re driving around Otter Tail County these bright summer days, it’s very likely that you’ll notice some patches of airy purple flowers with greyish-green leaves growing in road ditches and fields. Blooming June through August, spotted knapweed is especially noticeable this time of year. And because it likes our sandy soils, it is pretty well-spread throughout Otter Tail County.

Because spotted knapweed is on the Minnesota Noxious Weed list under the “control” category, it is legally required for knapweed to “be controlled in a way that prevents spread of these species by seed or vegetative means.” (https://www.mda.state.mn.us/plants-insects/minnesota-noxious-weed-list)

What does this mean for you as a landowner in the State of Minnesota and Otter Tail County? If you have spotted knapweed on your property, the law requires you to control it, whether through biocontrol methods, mowing, or herbicides. Why? Because spotted knapweed can spread very rapidly and outcompete other species, creating a monoculture. It actually exudes a chemical that prevents other plants from growing. This is bad for livestock AND bad for our wildlife. It can also be bad for you, as it can be a skin irritant to some people.

In order to effectively control spotted knapweed, it can be helpful to understand it’s life cycle and how it spreads. As a biennial, it grows low to the ground it’s first growing season and works on establishing it’s long taproot. On its second year of growth, the plant sends up the flower, which turns to seed. For smaller patches, continuous pulling can be an effective control method. For larger patches, mowing can also be effective, but because knapweed will continue to flower all summer long, it must be mowed all summer long to prevent the flower from turning to seed. Late spring and early fall applications of herbicide can also be very effective. Follow up treatments the next year will likely be necessary.

There are also a few biocontrol options, which can be very effective on large patches of knapweed. There are three biocontrol agents (insects) out there that damage the knapweed plant enough to prevent it from spreading. For the insects to be effective, the infestation of knapweed needs to be a continuous patch of an acre or more, and it shouldn’t be on disturbed sites. It also takes a few years to finally see the effects of weevils, so site monitoring and patience are key.

So what should you do if you notice you have spotted knapweed growing on your property now? It’s not too late to pull or mow it, just make sure you’re keeping an eye on it over the summer and get after it again before it sets seed. In fact, in order to be compliant with the Noxious Weed Law, you need to show that you’re trying to control it, and mowing is a great way to do that. Follow all of that mowing with an herbicide application in the fall to target those rosettes, and your work next year should be significantly easier. It may take a few years to effectively control it, but it will be well worth the hard work!

Posted In: Noxious Weed Control