Russian knapweed (Rhaponticum repens) Why you should care
Published on Jan. 18, 2020
Russian knapweed is a long-lived perennial plant, meaning it has a root system that survives over the winter and regrows each spring. These roots also spread beyond the original plants and are able to create “new” plants further from the introduction.
This “creeping” nature means this plant can spread into a dense mat (Image 1) without ever producing seed, although it does still produce seed that can be carried on the wind!
This perennial root system not only acts as a spread mechanism, it also releases chemicals into the soil that can injure other plants. This ability, termed allelopathy, is thought to be one of the reasons this invasive plant has become so successful in the United States. This ability further increases the damage it does to the ecosystem by reducing the biodiversity in the locations is invades.
Although not well studied, there is also a concern that these chemicals may persist in the soil even after Russian knapweed has been removed resulting in a greater challenge in trying to re-establish vegetation.
Spotted knapweed is also believed to produce chemicals that are toxic to some livestock, particularly horses. This toxicity also leads to reductions in wildlife activity in areas that are heavily invaded.
Key identification characteristics
First and most fortunately, there are no native knapweeds. Thus, as long as a suspect plant can be identified confidently as a knapweed, it is likely one of the three common invasive knapweeds, Russian, spotted and diffuse, of Wyoming and should be managed irrespective of the specific species. However, because the biology of these three invasive species are different, it is still a good idea to identify down to the species level.
Knapweeds have very variable leaf shapes, however, the texture of knapweed leaves is tacky and the color tends to be blueish in hue due to white hair like protrusions from the leaf surface. All knapweeds form clusters of white to purple flowers that look somewhat familiar to a Canada thistle flower.
Once identified as a knapweed, differentiation of Russian knapweed is easiest when looking at the flower. At the base of all knapweed flowers are a cluster of “bracts” (figure 2). Russian knapweed bracts have a papery film around their margins, diffuse knapweed has small bards around the margin, and spotted knapweed has small black tips on its bracts (thus its namesake). You can also look at the roots.
First, as previously mentioned Russian knapweed is long-lived perennial. Other species of knapweed are annual to weak perennials depending on the environment. When pulled up, the roots of Russian knapweed are black to dark brown.
What to do
Like any targeted perennial plant, the goal is to not only injure the aboveground plant material, but to also target belowground structures, such as roots. This can be done by repeatedly disturbing the aboveground plant so it “wastes” resources regrowing or by directly targeting the belowground growth.
Any mechanical removal, such as hand pulling or mowing, can be effective at removing the aboveground biomass ideally during the flowering stage of growth. These measures would require many years of consistent follow-up to effectively stress the belowground biomass with any effect. Also consider when performing these activities that they may disturb the soil which oftentimes benefits Russian knapweedor possibly even other weeds.
Herbicides, unlike mechanical measures, are capable of directly injuring the root system. These so called “systemic herbicides” can be applied to the aboveground biomass but are capable of traveling to the root systems and causing damage. The most effective herbicide for Russian knapweed control in natural areas while avoiding severe damage to much of the surrounding vegetation is the broadleaf herbicide Milestone at six ounces per acre.
If possible, spot treatments that only target the plant can further reduce off-target injury. This control measure has been effective from early summer into the dormant fall season.
Although herbicide is relatively effective at controlling this species, a follow-up treatment the following season will ensure that any escaping survivors can be controlled and stop unwanted seed production. This activity can also double as a monitoring event where any potential new populations can be spotted and responded to.
There are no biocontrol strategies that are known to significantly reduce Russian knapweed stands; thus, they are not recommended for this species.
Dan Tekiela is the University of Wyoming Extension Weed Specialist. He can be reached at firstname.lastname@example.org.