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August 4, 2025

August Weed of the Month: Progress on Knotweed Mapping and Management

In 2018, there were 493 mapped knotweed infestations in Minnesota. Now, there are 3,374 mapped infestations.

By Monika Chandler, Minnesota Department of Agriculture -

Knotweeds (Polygonum x bohemicus, P. sachalinese and P. cuspidatum) are a group of tough to manage plants on Minnesota’s Noxious Weed List. Knotweeds are herbaceous perennial plants that grow approximately 7 – 15 feet tall. They can grow into concrete and asphalt resulting in damage to buildings and roads. They grow quickly by seed, fragments and underground stems that send up new shoots. This allows them to overtake other vegetation.

Across the state, many groups, organizations and individuals have come together to work on knotweed infestations. Together, there’s been a lot of progress with knotweed mapping and management.

Wild parsnip is commonly found along roadsides and in fields.

Download picture here

Mapping

In 2018, there were 493 mapped knotweed infestations in Minnesota. Now, there are 3,374 mapped infestations. It’s been a sustained effort by many landowners and partner organizations to document infestations including location, size and density. Most of the infestations are in the Duluth, Twin Cities, Red Wing, and Winona areas but there has been a recent rise in reports around Brainerd, Bemidji, and St. Cloud as more people learn about knotweeds.

What have we learned?

A University of Minnesota research team studied Minnesota’s knotweed genetics to determine species and hybrids, conducted herbicide trials and examined carbohydrate sequestration to learn the impact of treatments. For more information check out the article Tangled up in knotweed: the need for adaptive management and view the webinar Effective Management of Knotweed in the Midwest. This Minnesota Invasive Terrestrial Plants and Pests Center research was funded by the Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizens Commission on Minnesota Resources.

• Surveys and studies revealed a majority of plants are hybrid, many produce a lot of seed, and there are numerous male and female plants.
• Although not necessary - bending, breaking or cutting knotweed stems in the early summer can be helpful. The plant will expend energy to repair damage, and plants will be shorter making a late summer herbicide treatment easier. Leave all stems at the infestation. Moving them to a new location such as a compost pile may result in knotweed stems rooting to start a new infestation.
• Herbicide treatment targeting and timing is very important for management success and to avoid negative impacts to the environment and non-target species. Herbicide should be applied according to the label instructions and targeted on the foliage in the late summer or early fall. See this Knotweed lifecycle and treatment timing graphic.
• Herbicide treatments can be effective if repeated over multiple years.

Management recommendations were updated based upon new research in this Knotweed Factsheet.

Management

• Prioritize management for knotweeds growing along waterways and into buildings and structures. Knotweed seed and fragments spread by moving water can quickly start new infestations.
• We are seeing long-term success with sustained effort. Although we have the knowledge to manage knotweeds, we don’t always have the funding to sustain efforts and knotweeds can rebound.
• After successful herbicide treatments, there can be bare ground in the knotweed treatment area. Keep in mind that knotweed treatments will continue for multiple years, and you may again lose the vegetation.
• It can be hard to find small knotweed stems one or more years after herbicide treatment. One option is to do treatments every two years so there is more foliage to treat.
• There has been some success with tarping over infestations with a very heavy-duty landscape fabric for approximately 7 years instead of using herbicide. If successful, this method kills vegetation under the tarp and leaves a bare area. But knotweed will often send up shoots outside of the tarped area and it can grow through many tarp materials such as plastic.

Areas where knotweeds were treated with an herbicide the previous fall are without vegetation for several months. After grasses and other vegetation grow in the area, it’s harder to find the remaining knotweed stems.

Download picture here

Next steps

• There is an ongoing need for outreach and education. University of Minnesota Extension, including Master Gardeners, and many other partners are continuing to teach knotweed identification, reporting, and management.
• Biological control using a psyllid insect (Aphalaria itadori) was recently approved for field release in the United States, but it is still experimental. No established bioagent populations have been documented in the US at this time.
• Although biological control may become an important management tool for large infestations, we will continue to recommend conventional methods such as targeted herbicide treatments for Minnesota’s relatively small infestations are still recommended.

For more information on noxious weeds and invasive insects, listen and subscribe to Smarty Plants, a Minnesota Department of Agriculture podcast. 

MEDIA: Contact Brittany Raveill, MDA Communications, at [email protected] or 651-201-6131 for more information on Weed of the Month