Brown, diffuse, and meadow knapweeds - Early Detection Species

Field of brown knapweed in flower

 
 

Brown knapweed (Centaurea jacea), diffuse knapweed (Centaurea diffusa), and meadow knapweed (Centaurea x moncktonii) are all early detection species in Minnesota as they are of high concern and are not known to be widely distributed in the state. Any potential sightings should be reported to the Minnesota Department of Agriculture. These knapweeds are herbaceous plants that can form dense cover in prairies, pastures, and open habitats. Cattle and other animals avoid eating them so their abundance can cause large reductions in available food for grazing animals. There is concern that these knapweeds will hybridize with the more widespread spotted knapweed and produce new weed issues.

Caution: Cover your skin by wearing gloves and long sleeves when working with these plants as they can be a skin irritant for some people.

Description

Appearance

Brown, meadow, and diffuse knapweeds are biennial or perennial plants that can grow up to four feet tall. Plants have flowers that can range in color from white to dark pink. Brown, meadow, and diffuse knapweeds can be challenging to identify and hybrids are possible. If you think you have one of these species, send photos to Arrest The Pest for identification assistance from the Minnesota Department of Agriculture. View the "Meet the Knapweeds" brochure for a side-by-side comparison of the species.

Leaves and Stem

Seedlings germinate throughout the spring and form a clump of low-lying leaves close to the ground (called a rosette). The second year and following years, plants send up a flowering stalk with leaves that are alternate (come off the stem one at time at each leaf node). All three species have hairs on their leaves.
There are some leaf differences among the species:

  • Brown knapweed: Leaves have wavy edges and are not deeply lobed.
  • Diffuse knapweed: Leaves at the bottom of the plants have deep lobes, while leaves higher on the plant do not have lobes.
  • Meadow knapweed: Most leaves do not have lobes, but some have wavy edges or lobes.

Flowers

Clusters of tiny white to pink flowers make up the flower heads. Flower color is not a reliable way to tell the species apart as plants of any species may have white or pink flowers. There are bracts below the flowers that look like small leaves or scales. The bracts are the main way to tell the species apart.

  • Brown knapweed: Bracts are round and wide.
  • Diffuse knapweed: Bracts are pointed and have spines on their tips.
  • Meadow knapweed: Bracts are rounded and edged with fringe.

Seeds

All three species have small brown seeds.

Roots

All three species have taproots.

Biology

Brown and meadow knapweed are described as short-lived perennials or perennials. Diffuse knapweed is described as a biennial or short-lived perennial. The three species can vary in their preferred habitat, but are generally found in prairies, pastures, and other open, sunny habitats:

  • Brown knapweed: Moist, cooler sites.
  • Diffuse knapweed: Dry, disturbed sites.
  • Meadow knapweed: Moist to wet sites.

View the "Meet the Knapweeds" brochure for a side-by-side comparison

Origin and Spread

Brown, diffuse, and meadow knapweeds are native to Europe. Brown and meadow knapweeds were intentionally introduced to the United States as forage plants for cattle, but the plants are not very palatable or nutritious for cattle. Diffuse knapweed seeds were likely introduced as a contaminant in seeds of other species that were being imported to the United States. Brown knapweed is present in the western, midwestern and northeastern United States. Diffuse knapweed is widespread in the western United States. Meadow knapweed is most common in Washington and Oregon. All three species have limited distribution in Minnesota and are considered early detection species in the state.

Refer to EDDMapS Distribution Maps for current distribution of brown knapweed, diffuse knapweed, and meadow knapweed.

Don't be fooled by these look-alikes

  • Canada thistle, Cirsium arvense (invasive) – Canada thistle leaves are prickly while spotted knapweed leaves are not.
  • Spotted knapweed, Centaurea stoebe spp. micranthos (invasive) - The differences in the bracts below the flowers are key for separating out spotted knapweed from the other knapweeds.

View the "Meet the Knapweeds" brochure for a side-by-side comparison:

  • Spotted knapweed: Bracts have a dark, triangular mark on their tips.
  • Brown knapweed: Bracts are round and wide.
  • Diffuse knapweed: Bracts have spines on their tips.
  • Meadow knapweed: Bracts are rounded and edged with fringe.

 

 
Regulatory Classification

Brown, diffuse, and meadow knapweeds are Minnesota Department of Agriculture Prohibited Noxious Weeds on the Eradicate List meaning that the above and below ground parts of the plant must be destroyed. Additionally no transportation, propagation, or sale is allowed.

Threat to Minnesota
  • Brown, diffuse, and meadow knapweeds release chemicals into the soil that can harm other plants (allelopathic) and form dense monocultures that threaten a variety of habitats including prairies, grasslands, savannas, dunes, sandy ridges, and forest openings.
  • They spread rapidly along road corridors and in gravel pits, agricultural field edges, and overgrazed pastures.
  • They reduce forage quality in pastures for livestock.
  • There is concern that brown, diffuse, and meadow knapweeds can hybridize with each other or spotted knapweed and create new weed issues.
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. The knapweeds have deep taproots 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 these plants because they can be skin irritants for some people. Follow Minnesota Department of Agriculture guidance for noxious weed disposal.

Mowing before seed has developed can help prevent seed spread. Mow from April to June to prevent flowers from forming. Do not mow if seed is present (generally June 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. The knapweeds are rosette of leaves in their 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.

Reporting

Report new occurrences by submitting a report through EDDMapS Midwest, emailing Arrest The Pest, calling Arrest the Pest (1-888-545-6684), or contacting your local county agricultural inspector.

Resources