PO Box 88, Manistee, Mi 49660

231-723-6996 or 231-845-2500 jamesscarlata@hotmail.com

Knapweed

Knapweed

Spotted knapweed (Centaurea stoebe) is an invasive plant native to Southeastern Europe.  Diffuse knapweed (Centaurea diffusa) is also invasive in Michigan.  Knapweeds are perennials, living two or more years, and beginning flowering in the second year.   Knapweeds invade dry prairies, meadows, and  dunes, as well as agricultural fields and roadsides.  Knapweeds kill other species of plants around them by releasing chemicals into the soil.  This process is known as allelopathy.

Knapweed left untreated causes significant damage that can include:

  • Loss of native plants that wildlife depend on for food and cover.
  • Increased run-off and soil erosion.
  • Reduced water quality due to increased run-off.
  • Decreased recreational value of land, as knapweed is prickly to walk through.
  • Toxic to cattle that graze on it.

Knapweed for web

Spotted  Knapweed in bloom.

Knapweeds are most easily noticed when they are in bloom in mid to late summer.  They have a thistle-like flower.  The flower of spotted knapweed is light purple, there is a head underneath the flower with dark spots.  Diffuse knapweed has a white flower with a spinier head.  The flower stalks of knapweeds can range from 6 inches to 36 inches tall.  In the spring the knapweed will be a rosette of green deeply lobed leaves.

Knapweed for web 2

Rosette of spotted knapweed in spring

Options for control of knapweeds:

  • Ideally, find new infestations before they set seed and hand pull or spot spray with a herbicide for broadleaf weed control. Wear gloves when hand pulling as knapweed can cause skin irritation.
  • Mowing when plants are flowering but before seeds form can reduce seed spread. Clean your mower after mowing areas where there is mature knapweed seed to prevent spreading it to new areas.
  • Sheep and goats can graze on knapweed, but may hinder the reestablishment of desireable plants as well.
  • Several species of insects that feed on knapweed are under study in Michigan.
  • Chemicals that selectively target broadleaf weeds can be used to kill large patches of knapweed. Be sure the chemical you select does not cause damage to adjacent trees that may have roots under the treated area.  Follow all safety precautions on herbicide labels.

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