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Home>Plant Species>Weed Species>Spotted knapweed

Drawing of spotted knapweed Spotted knapweed
Centaurea biebersteinii (Centaurea maculosa)

question markDid you know?

Spotted knapweed seeds can remain dormant for 5 years!

How did spotted knapweed get here?
Spotted knapweed ranks as the number one weed problem on rangeland in Montana and is reported to occur in all but 5 states. Northwestern states have experienced the hardest hit from spotted knapweed to this point. It was introduced from Eurasia as a contaminant of alfalfa and clover seed and/or soil used in ship ballast in the late 1800s.

What are its characteristics?
Spotted knapweed is a perennial forb, but doesn’t live much over 10 years. It can have one or more stems and grow 1 to 3 feet tall. Pink to purple flowers appear at ends of branches from June to October. Spotted knapweed flowerheadsThis species reproduces by seeds which can remain dormant for 5 years, waiting to germinate when conditions are favorable. A compound in spotted knapweed called cnicin has antimicrobial properties, and is active against some human cancers.

Why is Spotted knapweed a problem in the Great Basin?
Spotted knapweed creates dense stands that crowd out more beneficial species. For wildlife, spotted knapweed is considered more detrimental to elk than deer because it replaces grasses that are preferred by elk, while deer eat more shrubs and other browse. Large reductions in available forage and wildlife use have been reported on infested range; however, that has changed some since studies have shown that it has good nutritional value and it is eaten by both livestock and wildlife. A large, perennial taproot helps spotted knapweed survive fires. It also produces large quantities of seed that can thrive without competition from other plants following fire.

How can we fight this weed?
As with all weeds, preventive practices begin by maintaining vegetation that is fairly resistant to weed establishment. This includes minimizing soil disturbance and reestablishing desirable vegetation whenever disturbance occurs. Regular removal of new spotted knapweed plants at trailheads, campsites and along road corridors is critical to prevent spread to new areas. Certified weed-free feed should be fed to animals for several days before entering the backcountry. Spotted knapweed plant


A key component of prevention is education and awareness of managers, land owners, and public land users. In Montana, cooperative weed management programs have been implemented for landowners, and a curriculum has been designed for educating school children.

Many biological control agents have been studied, and it is heartening that several flies moths, and weevils have shown some promise. Sheep grazing has been proposed as a potential control method as well. For small-scale control, herbicide application or pulling and digging by hand are effective.

Images:
-Drawing: USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. Illustrated flora of the northern states and Canada. Vol. 3: 558.
-Flower: Intermountain Herbarium.
-Plant: Colorado State University Herbarium.

Text references:
-Invasive Plants of the Eastern United States. www.invasive.org (http://www.invasive.org/eastern/biocontrol/13Knapweed.html 27 September 2006).
-USDA, NRCS. 2006. The PLANTS Database (http://plants.usda.gov, 26 September 2006). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
-Whitson, T. D., et.al. 2000. Weeds of the West., 9th ed. Western Society of Weed Science, Newark, CA.
-Zouhar, Kris 2001. Centaurea maculosa. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: (http://www.fs.fed.us/database/feis/, 26 September 2006).