Centaurea virgata (Centaurea squarrosa)
Did you know?
Squarrose knapweed likes dry
rangelands with shallow soils!
How did squarrose knapweed get here?
Overall, knapweeds are a large group of over 400 species, many of which are considered invasive weeds. Squarrose knapweed is native to the eastern Mediterranean area. How it was originally introduced is unknown. It became weedy in northern California and Utah by the early 1950’s, and its spread was associated with ranging sheep. While not yet widely distributed in the West, where it has gained a foothold, it has become a severe problem.
What are its characteristics?
This long-lived perennial forb typically reaches a height of 12 to 18 inches. Stems have multiple branches. Flower heads are relatively small, usually developing no more than 3 to 4 seeds per head. It is often confused with diffuse knapweed, but differs in that squarrose is a true perennial, has smaller flowerheads, its bracts on the flowerhead are recurved, and seed heads fall off the stems soon after seeds mature.
When conditions are unfavorable, plants may remain as rosettes for several years before developing a flowering stem.
Why is squarrose knapweed a problem in the Great Basin?
Squarrose knapweed invades dry rangeland with shallow soils. It can survive harsh climates, and is also tolerant of fire. Like other knapweeds, squarrose knapweed competes with forage species on rangeland and reduces forage production for livestock and wildlife. The flower heads of squarrose knapweed function like burs clinging to passing animals, vehicles, and clothing.
How can we fight this weed?
Seeds can get caught underneath a vehicle when driven through a squarrose knapweed patch, and so this should be avoided. Livestock should not graze infested areas after seeds have matured. It is important to detect the presence of knapweed plants early, before they become large populations. Controling outlying plants before attacking large populations is usually more effective. Stout taproots resprout when broken off, making hand pulling ineffective. Instead, individual plants should be cut at least eight inches below the soil surface to prevent resprouting.
Control of squarrose knapweed with herbicides must be used with other control methods to be successful. Many biological control insects that attack other knapweeds also accept squarrose knapweed as a host. Refer to the fact sheet on squarrose knapweed control from the University of Nevada and the Weed Management Handbook on the University of Wyoming Extension website for additional information. After any control is applied, establish desirable plants to avoid reinvasion.
-Drawing: Michael B. Piep © 2006
-Flower: Oregon State University Jed Colquhoun photo Collection.
-Plant: Steve Dewey, Utah State University..
-USDA, NRCS. 2006. The PLANTS Database (http://plants.usda.gov, 26 September 2006). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
-University of Nevada Reno. Cooperative Extension. Fact sheet 4-38. Available:(http://www.unce.unr.edu/publications/FS04/FS0438.pdf 27 September 2006).
-Whitson, T. D., et.al. 2000. Weeds of the West., 9th ed. Western Society of Weed Science, Newark, CA.