Spotted Knapweed

August’s Weeds of the Month: Field Bindweed & Spotted Knapweed

Weed of the Month, Number 1: Field Bindweed

Field BindweedField bindweed, a native of Eurasia, is thought to have been introduced into the US through contaminated seed as far back as 1739. This vine species forms a monoculture by climbing and twisting its way up anything and everything keeping other plants from reaching light.

Field Bindweed
Field Bindweed

Field bindweed is a rhizomatous perennial. Its roots can reach down nearly 14 feet in its search for moisture. As the roots grow away from the parent plant, new shoots are formed creating new plants. This species can grow up to 10 feet in a growing season. Each of its morningglory flowers produce two seeds which may be viable for more than 60 years!

Because field bindweed is a rhizomatous perennial, hand pulling is not an effective control option. Heavy cultivation may be effective if repeated frequently throughout the growing season. Solarization may have limited success because the temperatures required for control are difficult to sustain in our environment. Herbicides are also an effective part of any integrated control plan for field bindweed.

Field Bindweed, what to look for?
  • Deeply rooted perennial vine that typically grows along the ground until finds a structure or other plant to climb
  • Smooth, arrowhead-shaped leaves
  • Long, slim, winding stems
  • Trumpet-shaped flowers, typically light pink to white
  • Two small leaves about an inch below the flower
  • Pale roots

Weed of the Month, Number 2: Spotted Knapweed

Spotted knapweed, like many of the invasive species in Teton County, originated in Eurasia. It was first identified in the US in the late 1800s in Washington, and it is thought to have been a stowaway in a shipment of alfalfa. It forms dense stands by both out competing other species and by exuding a chemical into the soil that inhibits germination of  neighboring plants. It’s primary means of spread is its seeds, which are dispersed by wind, gravity, and animals.

Spotted Knapweed
Spotted Knapweed

Spotted knapweed is a taprooted, biennial species. In its first year of growth rosettes are formed, but it does not begin to form flowers until its second year of growth. In some instances, spotted knapweed may take on more a perennial habit and grow a third season.

Because spotted knapweed has a taproot, it can be effectively managed by hand pulling; however, gloves and long sleeves are recommended because it can cause skin irritation. Dense infestations may be best controlled with herbicides to reduce soil disturbance.

Spotted Knapweed, what to look for?
  • Resembles a thistle but has no spines
  • Vertical branched stems, can reach heights of 5 feet tall when flowering
  • Flower heads are small and oval
  • Light purple to brilliant pink thistle flowers
  • Leaves at base of thistle-like flower (called bracts) have triangular black spots
  • Leaves: medium-green with a silvery gray