Spotted knapweed identification and control

Be on the lookout for this noxious weed and get rid of it. See the Missouri Revised Statute 263-232 Eradication and control of the spread of teasel, kudzu vine, and spotted knapweed.

Click on thumbnails below to view a larger photo.

photo: spotted knapweed growing in a field

Spotted knapweed from a distance in June

photo: close up of the rosette stage without a flower

Close up view of rosette stage in early November

photo: spotted knapweed plant size in late April

Plant size in late April

photo: flowering stage of spotted knapweed

Flowering stage in late June
(note black-tipped flower bracts)

photo: close up of the flower

Close-up view of flower size

photo: close up of root borer weevil (cyphocleonus)

Cyphocleonus (root borer weevil)

Description, location and control options

Spotted knapweed (Centaurea stoebe micranthos), a weed native to Europe and introduced to the U.S. in the late 1800s, has made its way to Missouri. First spotted in southwest Missouri in 2002, this noxious weed is now found throughout southwest and south-central Missouri mostly south of Interstate 44, as well as two multi-county areas of east-central and north-central Missouri.

Map of Missouri counties infested with spotted knapweed (JPG)

Road-by-road map of infestations statewide in 2009 (PDF)

Spotted knapweed is a short-lived perennial plant that grows 2-4 feet tall. It forms a rosette the first year and then sends up a flower stalk the second year. The leaves, which are rather sparse on a hard and woody stem, have a pale green color. The weed blooms in mid-summer, and the flowering bud is about 3/4-inch long. The blooms are pink to purple in color and rather attractive. The roots of this weed produce a toxin that kills other plants within its root zone. The weed reproduces solely by seeds, and the weed is a prolific seed producer, with 1000 or more seeds per plant. Once established, seeds accumulate in the soil, often exceeding 5000 per square foot. The seeds remain viable for at least eight years.

Landowners are advised to be watchful for spotted knapweed, and keep it in check while it is only along roads and not out in pastures. If not controlled, it can rapidly invade pastures and fields and cause a serious decline in forage and crop production.

Several herbicides are effective at controlling spotted knapweed when used in a timely manner. In order of effectiveness are:

  • Milestone at a 5 to 7 ounce per acre rate (based on plant size)

  • Tordon 22K at 1 pint per acre (restricted-use herbicide)

  • Grazon P+D at 4 pints per acre (restricted-use herbicide)

They are best applied at the bud stage in the spring or at the rosette stage in the fall.

Once the plant blooms, herbicides are rather ineffective. The plants should then be pulled up and burned, but landowners are advised to wear gloves to avoid skin irritation from the weed contact.

Biological control, begun in 2008 and expanded in 2009, involved the release of two weevils that are host-specific to the spotted knapweed. Seedhead weevils (Larinus minutus/obtusus) were released in June-July, and root borer weevils (Cyphocleonous achates) were released in August. Knapweed seedhead flies (Urophora quadrifasciata) that feed on spotted knapweed are already present in Missouri.

It will take several years for populations of these insects to grow enough to begin providing significant control of the spotted knapweed. Locations of the releases made by the Missouri Department of Transportation, Missouri Department of Conservation and University of Missouri Extension are shown on the county maps below.

County maps showing locations of releases of biological control insects:

For more information, contact one of the University of Missouri Extension agricultural specialists or references below:

Barton County
Jay Chism
chismj@missouri.edu
417-682-3579

Lawrence County
Eldon Cole
colee@missouri.edu
417-366-3102

McDonald County
John Hobbs
hobbsj@missouri.edu
417-223-4775

Stone
Tim Schnakenberg
schnakenbergc@missouri.edu
866-357-6812

Webster County
Bob Schultheis
schultheisr@missouri.edu
417-859-2044

References