Posted on September 5, 2009 - by mckenzie

Noxious Weeds Part I: Spotted Knapweed

Spotted Knapweed

Spotted Knapweed is an aggressive, introduced weed species that rapidly invades pasture, rangeland and fallow land and causes a serious decline in forage and crop production. The weed is a prolific seed producer with 1000 or more seeds per plant. Seed remains viable in the soil five years or more, so infestations may occur a number of years after vegetative plants have been eliminated. Spotted Knapweed has few natural enemies and is consumed by livestock only when other vegetation is unavailable. The plant releases a toxin that reduces growth of forage species. Areas heavily infested with Spotted Knapweed often must be reseeded once the plant is controlled.

Historical records indicate that Spotted Knapweed was introduced from Eastern Europe into North America in the early 1900s as a contaminant in crop seed. It now infests several million acres of grazing land in the northwestern United States and Canada.

 Identification:

Spotted Knapweed is a biennial plant that germinates in one growing season, then flowers, seeds, and dies during a second or short-lived perennial plant that lives for more than 2 growing seasons. Mature plants may be 3 feet in height and are much branched. The weed forms a basal at the base of plant or plant part rosette, a circular, normally basal, clump of leaves. The first year and stem leaves are pinnately with 2 rows of leaflets, like a feather divided. Flower heads are abundant, ½ inch wide, and generally solitary on branch tips. Flowers are pink to purple, or occasionally white, and appear from midsummer to fall. Each stiff flower head bract leaflike structure at the base of flowers or leaves has a dark comblike fringe resembling a black spot at the tip. Seeds are dark brown to tan and are tipped by plumes. A hairlike or featherlike structure, often on a seed that fall off at maturity.

Control:

Spotted Knapweed is one of the most prolific noxious weeds in Idaho. Several biological control agents are available to stop the spread of this plant. A seed head moth and 2 seed head gall flies have been effective. Effective herbicides are also available for control of Spotted Knapweed. For more information or to report a large crop of Spotted Knapweed, please call the Idaho Weed Awareness Campaign at 1-866-IDWEEDS. You can also log on to Idaho’s Virtual Weed Guide to find out more about noxious weeds in your area: Click here for the Virtual Weed Guide.