ENVS 605

 

What is it?

Spotted Knapweed is a short-lived perennial (typically 3- 7 years). It reproduces from seed and forms a new shoot each year from a taproot. As many as 40,000 seed have been found on one plant. It is found throughout Colorado in dry meadows, pastures, rocky hillsides, roadsides and floodplains. It is similar in appearance to diffuse knapweed. The main differences are that Spotted Knapweed has one or more stems (Diffuse Knapweed only has one stem) and  the black-tipped involucral bracts at the base of the flower. Spotted Knapweed is designated as a “List B” species on the Colorado Noxious Weed Act. It is required to be either eradicated, contained, or suppressed depending on the local infestations. It was introduced to the Americas from Eurasia as a contaminant in alfalfa and clover seed and through ballast dumps.

 

Why is it a problem?

Spotted Knapweed can tolerate both wet and dry conditions, making it a versatile invader of disturbed and over-grazed areas. Reduced livestock and wildlife forage are the result of its ability to out-compete native species. It increases surface runoff, erosion and stream sedimentation. The sap can cause skin irritations in some humans. Vectors for spreading include people, livestock, vehicles, crop seed and contaminated hay. Common seed distribution facilitators include gravel pits, soil stockpiles, powerlines, grain elevators and railroads.

 

Management Techniques:

• Spotted Knapweed can be effectively controlled with herbicide. Tordon, Milestone, Transline and Curtail are the common herbicides used.

• Cultural techniques (seeding suitable grasses, forbs and shrubs) must be employed to eliminate reinvasion.

• Livestock will eat spotted knapweed and have been used to decrease seed set.

• Additional biological controls, usually less effective, include two weevils (Cyphocleonus achates and Larinus minutus). They attack Spotted Knapweed’s root system and reduce seed production.

 

What can we do?

Prevent seed production and establishment with proper land management. Digging can be effective if done when the soil is moist. Be sure to remove not only the taproot, but all lateral roots as well. If cutting or mowing, make sure to bag cut plants that are flowering as the seeds will still be viable. Avoid traveling through infested areas and if you do, clean footwear, clothing, backpacks, etc. Use only weed free hay.
 

General Information on Spotted Knapweed:

National Forest Service

• Website on control methods for spotted knapweed

 

National Park Service

• PDF, fact sheet on spotted knapweed

 

Colorado State University Extension

• PDF, identification and management of spotted knapweed

 

Colorado Weed Management Association

• Website on the identification, biology and integrated management of spotted knapweed.