Spotted knapweed, a weed that produces a toxin via the roots that kills plants within its root zone, has made its way to Webster County. First spotted in southwest Missouri in 2002, this noxious weed is now found along all major highway and railroad rights-of-way in Webster County. The heaviest concentrations are along Highway 60, Highway B, Highway A, Highway C north of Seymour, and Highway 38 east of Marshfield.

Click here for a map of infestations statewide.

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 weed reproduces solely by seeds, and the weed is a prolific seed producer with 1000 or more seeds per plant.

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-ounce per acre rate, Tordon 22K at 1 pint per acre, or Grazon at 4 pints per acre. 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 controls using a root borer weevil, a flower head weevil and a fly are being tested, but it will be several years before effectiveness of this approach is known.

For more information, contact:

Webster County Extension Center
Attn: Bob Schultheis, natural resource engineering specialist
800 S. Marshall St. (next to City Hall)
Marshfield, MO 65706
Phone: 417-859-2044
E-mail: websterco@missouri.edu

Hickory County Extension Center
Attn: Brie Menjoulet, agronomy specialist
P.O. Box 187
Hermitage, MO 65668
Phone: 417-745-6767
E-mail: menjouletb@missouri.edu

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