Photo: Common tansy by Joshua Heath on Unsplash.com

By Laura Durenberger-Grunow - Boreal Community Media - June 5, 2026

Caring for our local landscapes is a shared community effort, and keeping an eye out for invasive plant species is a normal part of maintaining healthy properties. In Cook County, community members frequently encounter three specific invasive, non-native plants in their yards and neighborhoods: creeping bellflower, Canada thistle, and common tansy.

Managing these plants is not only ecologically beneficial for protecting our native habitats but, in some cases, also a legal requirement for landowners.

The state and county monitor certain plants to protect local ecosystems. Earlier this year, an official general notice to control or eradicate noxious weeds was posted by the county in the Cook County News Herald. Both Canada thistle and common tansy are officially listed as Minnesota Noxious Weeds and are classified as "Prohibited-Control." According to Cook County Environmental Technician and Cook County Invasives Team member Abigail Prom, this means that "efforts must be made to prevent seed maturation and dispersal of plants into new areas." Additionally, regulations prohibit the transportation, propagation, or sale of these plants. Failure to comply with these rules may result in enforcement action by the county or local municipality.

When looking at these particular species, Prom said that they all share similar biological traits, meaning that managing them successfully comes down to understanding how they grow and utilizing straightforward, practical strategies. This is because all three species propagate by producing seeds and spreading via rhizomes, or underground roots.

Management efforts at home

The main goal for all three plants is to prevent them from going to seed. For example, creeping bellflower can produce up to 15,000 seeds per plant after flowering, according to the U of M Extension Office. 

Preventing plants from going to seed can be done by consistently and repeatedly hand-pulling, digging up the roots (the goal is to remove all roots if possible), and/or mowing (you can safely mow until seeds appear on the plant; after seeds appear, mowing does more harm than good). 

After pulling or digging, Prom said the best practice is to leave the pulled plants on-site to dry out in the sun. Placing the plants on a tarp or a plastic bag helps speed up the process. From there, the plants can be burned (check for any burn restrictions beforehand) or left in a bag to decompose. According to U of M Extension, it's illegal to place yard waste, such as weeds, in trash containers. 

Another option, especially for large infestations, is to place newspaper, cardboard, or a tarp over the plants to smother them. 

Sometimes management efforts must be carried out over multiple seasons. For persistent infestations, Prom said, common tansy and Canada thistle both respond well to foliar herbicide applications. Spraying the rosettes (the small, basal plants not in flower) is most effective when completed in the spring and fall, she added. 

Community Invasive Species Management Program

The Cook County Invasives Team offers a program called the Community Invasive Species Management Program, which connects invasive species management tools in the hands of community members and property owners at a low cost. The program includes herbicide safety training and an herbicide mix. Additionally, community members can access a backpack sprayer, hand sprayer, loppers, safety glasses, and glove rental for $20.  For those who have their own equipment, the Invasives Team can provide advice on a management plan. 

To be eligible for the program, landowners must have a Minnesota 'Prohibited Noxious Weed' on their property, including common tansy, Canada thistle, garden valerian, spotted knapweed, knotweed, purple loosestrife, wild parsnip, leafy spurge, dalmatian toadflax, or others. The full species list can be reviewed on the Minnesota Department of Agriculture's noxious weed webpage.

4th Annual Invasive Species Brewery Event

To learn more about invasive species in Cook County, the Invasives Team is hosting its 4th annual brewery event at Voyageur Brewing in Grand Marais on June 20, from 2 p.m. to 4 p.m.

The event will feature family-friendly activities, invasive species educational booths, and more. Additionally, invasive species trivia (with prizes!) will start at 2:45 p.m. 

As with past events, attendees can enjoy a limited-release, mystery SnALE Ale. According to the Cook County Invasives Team, the first 100 guests to purchase a pint of the new beer will receive a limited-edition pint glass featuring a "Keep 'Em in the Tank - No Lake Dumping" mystery snail graphic.

To learn more about the Cook County Invasives Team, click here.