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KNAPWEED

Russian Knapweed
Acroptilon repens L.


Russian knapweed is the most wide spread of the knapweeds in North Dakota and infested approximately 3500 acres in 1997. It also is the only perennial of the noxious knapweeds and is the most difficult to control. The largest infestations generally are found in southwestern North Dakota. Russian knapweed is adapted to poorly drained and saline/alkaline soils. It is often found in areas with a supplemental water source such as the Little Missouri and Heart Rivers in North Dakota. Russian knapweed will also infest roadsides, pasture, and rangeland and is the only knapweed in the state that causes significant losses in cropland.

Russian knapweed is a long-lived, deep-rooted perennial with growth characteristics similar to Canada thistle. The weed emerges in the spring from roots and grows 1 to 3 feet tall (Figure 2). Once established, Russian knapweed spreads mainly by underground root stocks as seed production is limited compared to other knapweed species. Two key characteristics distinguish Russian knapweed from spotted and diffuse knapweed.The flowers have rounded bracts with transparent tips (Figures 3 and 4). The flowers of Russian knapweed vary from light pink to lavender (Figure 3). Flowering occurs from June to September.    Top 

Figure 2.  It is very difficult to distinguish knapweed species based on the rosette alone. Spotted knapweed is deeply divided with wide lobes (top), diffuse knapweed has more finely divided lobes similar to carrots (middle), and Russian knapweed is seldom divided, has a "rabbit ears" appearance, and is a perennial (bottom).



Figure 3.

Figure 3. Spotted knapweed has stiff black tip bracts with purple flowers (left), diffuse knapweed has both purple and white flowers and bracts with rigid sharp spines (center), Russian knapweed has pink to purple flowers with opaque bracts and the flower heads are generally larger than the other two species (right).


Figure 4

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The root of this perennial is dark brown to black in color, scaley as if the plant had been burned, and can grow to depths of greater than 20 feet (Figure 6).

 

 

 

 

 

                                               

Figure 6. The roots of spotted and diffuse knapweed are taproots similar to dandelion and off-white in color (left). Russian knapweed roots are brown to black in color with a scaley, bark-like appearance and because it is perennial have root buds (right).

 

 

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Prevention

People are the major cause of knapweed spread. Knapweeds are spread readily in hay and on vehicle undercarriages. Producers should exercise caution when using hay from road ditches and when purchasing hay from known infested areas in neighboring states and provinces.

Land managers must learn to identify knapweed on their own and neighboring land, especially on disturbed sites, pastures bordering roads and streams, and where hay is fed. Timely control of a few plants will be very cost-effective compared to treating larger acreage later. The public can assist county weed officials in controlling knapweeds by reporting all suspected infestations. Top

Control

Russian knapweed is one of the most difficult perennial weeds to control. If the plant is found in cropland, then a combination of cultivation and herbicide treatments will suppress the plant. However, herbicides at labeled rates for cropland use will not control Russian knapweed.

Small patches in pasture and rangeland. The same herbicides, except 2,4-D, used for spotted and diffuse knapweed will control Russian knapweed when applied at higher rates. Picloram (Tordon) should be applied at 0.5 to 0.75 pounds (2 to 3 pints) per acre, dicamba (Banvel) at 2 to 3 pounds (2 to 3 quarts) per acre, and clopyralid plus 2,4-D (Curtail) at 0.38 plus 2 pounds (4 quarts Curtail) per acre. In addition, metsulfuron (Escort) plus 2,4-D is labeled for Russian knapweed control at 0.6 ounces plus 0.5 to 1 pound (1 oz Escort plus 1 to 2 pints of a 4-pound-per-gallon concentrate 2,4-D) and should be applied with a non-ionic surfactant. Metsulfuron has no grazing restrictions. The optimum application time for any of these treatments is when Russian knapweed is in the bud to early bloom growth stage or in the fall following a light frost.

Biological Control

In general, the knapweed infestations are small enough that herbicide and hand removal are the best and most cost-effective treatments in North Dakota. Biological control of various knapweed species in neighboring states has not been successful in reducing established knapweed stands. Biocontrol agents have not been introduced into North Dakota and their use is not recommended.

The best knapweed control is prevention, and to keep an infestation from becoming established you must correctly identify the plant.

Small and young infestations of knapweed are generally easy to control with herbicides. However, an area must be monitored for several years and retreated as necessary for seedling control.

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Photos 1, 2, 5 and 6 by Rodney G. Lym.
Photos 3 and 4 courtesy of Dr. Steve Dewey, Utah State Univ., Logan.

 

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