Controlling Spotted Knapweed in Michigan

Spotted knapweed (Centaurea stoebe) is a significant invasive species concern in Michigan. Spotted knapweed is a perennial plant that grows 2 to 3 feet in height. It has long blue-gray-green leaves divided into lobes. In July through September, it will have pinkish-purple flowers. It’s long tap-root can send shoots to start new plants. The seeds are carried on fine white fuzz. Spotted knapweed is found in open fields with poor sandy soils, in areas that have been disturbed and will take over pasture and hay fields. Spotted knapweed is poisonous to other plants, creating fields of only spotted knapweed. It can also be a skin irritant.

Effective management of spotted knapweed will require a persistent, comprehensive plan that combines multiple control methods. Here's a look at effective techniques for controlling spotted knapweed in Michigan:

Hand-pulling
For small infestations, hand-pulling remains one of the most effective and environmentally friendly control methods. To ensure success:

• Remove the entire root system, including the root crown (approximately 2 inches below the soil line).
• Wear gloves and long sleeves, as the plant can irritate sensitive skin.
• Bag and dispose of pulled plants to prevent seed spread.
• Implement this method before the plant sets seed to maximize effectiveness.

Herbicide Treatment
Chemical control is often necessary for larger infestations. The most effective herbicides for spotted knapweed control in Michigan include:

• Clopyralid: A selective herbicide that targets broadleaf plants while sparing grasses.
• 2,4-D amine: Another selective herbicide effective against broadleaf weeds.
• Glyphosate: A broad-spectrum, non-selective herbicide for areas where killing of grass plants is not a concern.

For optimal results:

• Apply herbicides when plants are actively growing, typically in spring or early summer.
• Target first-year rosettes for large infestations.
• Use a combination of products, which will help with control.
• Always follow label instructions, including rate recommendations and safety precautions.

Mowing
While not a standalone solution, mowing can be an effective component of an integrated management plan:

• Mow before flower set to reduce seed production.
• Use as a preparatory step before herbicide application or inter-seeding with native species.
• Combine with other methods for enhanced control.

Integrated Approach
For the best results, multiple methods of control should be used:

• Mow infested areas before applying herbicides.
• Follow the herbicide treatment with hand-pulling of survivors.
• Implement a long-term management plan that includes regular monitoring and follow-up spot treatments.

Burning
Prescribed burning can be an effective tool in spotted knapweed management:

• Reduces seed germination, plant survival, and flowering.
• Requires careful timing and sufficient fuel load.
• Most effective when combined with other control methods.
• Should be conducted by trained professionals following local regulations.

Prevention and Early Detection
Preventing new infestations and quickly addressing small populations is crucial:

• Clean equipment, vehicles, and clothing after visiting infested areas.
• Stay on established trails to prevent seed spread.
• Monitor vulnerable areas regularly for new infestations.
• Educate others about the importance of spotted knapweed control.

Long-term Management
Successful control of spotted knapweed requires an ongoing effort:

• Conduct regular follow-up treatments to manage regrowth and new seedlings.
• Monitor treated areas for at least several years after initial control efforts.
• Consider reseeding treated areas with native species to prevent reinfestation.

By implementing a comprehensive, integrated approach to spotted knapweed control, land managers and property owners in Michigan can effectively combat this invasive species. Early detection, prompt treatment, and persistent follow-up are key to success. As research continues and new control methods emerge, staying informed about the latest management techniques will help ensure the long-term health of Michigan's native ecosystems.

Young spotted knapweed plant. |Rob Routledge, Sault College, Bugwood.org

Spotted knapweed flower. |Rob Routledge, Sault College, Bugwood.org