Spotted knapweed (Centaurea stoebe spp. micranthos)

patch of spotted knapweed plants with pink/purple flowers

 
 

Spotted knapweed is a biennial or short-lived perennial herbaceous plant that grows two to three feet tall. It can form dense cover in prairies, pastures, and open habitats. Cattle and other animals avoid eating it so it can cause large reductions in available food for grazing animals. Biological control insects are used to help manage this plant.

Warning: Cover your skin by wearing gloves and long sleeves when working with this plant as it can be a skin irritant for some people.

Description

Appearance

Spotted knapweed is a biennial or short-lived perennial herbaceous plant that can grow 2-3 feet tall. Basal leaves form a rosette the first year from which grow one to twenty wiry, branched stems during the second year.

Leaves and Stem

In the first year, a clump of low-lying leaves forms close to the ground (called a rosette). In the second year, the plant sends up a wiry stem with alternate, grayish leaves that are deeply incised. Leaves decrease in size at the top of the stem.

Flowers

Thistle-like pink to purple flowers sit at the tips of the stems and bloom from July through September.

Seeds

Seeds are brownish, ¼ inch long, with small tufts of bristles. Seeds are dispersed by rodents, livestock, and in commercial hay. Seeds can stay viable in the soil for 7 years.

Rhizomes and Roots

Spotted knapweed has a stout taproot. Side shoots can form new rosettes near the parent plant.

Biology

Spotted knapweed is biennial or a short-lived perennial. A rosette of leaves is formed in the first year. They second year it grows stems and flowers. Seeds spread plants to new areas. Plants can also send up shoots to form new plants near the parent plant.

Origin and Spread

Spotted knapweed is native to Europe and Asia. Its seeds were likely introduced as a contaminant in seeds of other species that were being imported to the United States.

Refer to EDDMapS Distribution Maps for current distribution.

Don't be fooled by these look-alikes

  • Canada thistle, Cirsium arvense (invasive) – Canada thistle leaves are prickly while spotted knapweed leaves are not.
  • Other non-native knapweeds such as diffuse knapweed (Centaurea diffusa), meadow knapweed (Centaurea x moncktonii), and brown knapweed (Centaurea jacea). The differences in the bracts below the flowers are the main key for identification. View the "Meet the Knapweeds" brochure for a side-by-side comparison.
 
Regulatory Classification

Spotted knapweed is a Minnesota Department of Agriculture Prohibited Noxious Weed on the Control List meaning that efforts must be made to prevent the spread of seeds or other propagating parts. Additionally no transportation, propagation, or sale is allowed.

Threat to Minnesota
  • Spotted knapweed is poisonous to other plants (phytotoxic) and forms dense monocultures.
  • It especially threatens dry prairie, oak and pine barrens, dunes, and sandy ridges.
  • It spreads rapidly along road corridors and in gravel pits, agricultural field edges, and overgrazed pastures.
  • Spotted knapweed reduces forage quality in pastures for livestock.
What you should do

One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.

PlayCleanGo: Stop Invasive Species in Your Tracks

  • REMOVE plants, animals & mud from boots, gear, pets & vehicle.
  • CLEAN your gear before entering & leaving the recreation site.
  • STAY on designated roads & trails.
  • PLANT non-invasive species.
Native Substitutes
 
Control Methods

Mechanical control can be done by digging or pulling the plant by hand or with equipment such as a sharp shovel. Spotted knapweed has a deep taproot so care must be taken to remove as much of the taproot as possible. Cover your skin by wearing gloves and long sleeves when working with this plant as it can be a skin irritant for some people. Mowing before seed has developed can help prevent seed spread. Mow from May to July to prevent flowers from forming. Do not mow if seed is present (July and later).

Herbicide control can be done using systemic herbicides. Systemic herbicides are taken up by plants and move within the plant, which can kill leaves, stems, and roots. Herbicide formulations including aminopyralid, clopyralid, glyphosate, imazapyr, aminocyclopyrachlor, and picloram have been found to provide good control. Always follow label instructions when working with herbicides. Spotted knapweed is a rosette of leaves in its first year. Herbicide treatments are most effective when applied to this leaf rosette stage. Fall can be a good time to find and target the rosettes. Herbicides are also effective on the stem bolting stage of the plant in spring of its second year. Use caution and specifically target the herbicide to knapweed plants to avoid damage to non-target plants.

Biological control in Minnesota is done using two seedhead weevils (Larinus minutus and L. obtusus) and a root-boring weevil (Cyphocleonus achates). The larvae of seedhead weevils eat developing spotted knapweed seeds and reduce the number seeds that spotted knapweed plants produce. The seedhead weevils are widely distributed in Minnesota. The larvae of root-boring weevils eat the roots of the plants and weaken or kill the plants. Root-boring weevils are not as widely distributed in Minnesota as the seedhead weevils. For more information about spotted knapweed biocontrol, visit the Minnesota Department of Agriculture spotted knapweed biocontrol webpage and contact your local county agricultural inspector.

Reporting
Resources