Establishing Desirable Grasses in Russian Knapweed Infested Sites

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Russian knapweed (Acroptilon repens) is a creeping perennial that spreads through seed and vegetative root buds. Once established, infestations expand primarily by adventitious roots.

Russian knapweed is common throughout the western United States, infesting about 1.2 million acres of rangeland, cropland, pastures, and disturbed sites. The weed is poisonous to horses but is usually avoided by grazing animals. Dense monocultures of Russian knapweed reduce desirable vegetation through a combination of competition and allelopathy. 

“The extensive root system makes long term control of the weed more difficult,” explained Joe Vassios, a graduate student at Colorado State University. “In addition, there is speculation that allelopathic residues in surface plant material or root exudates can impact successful establishment of desirable grass following Russian knapweed removal.”

Vassios, in cooperation with Dr. Scott Nissen, Dr. George Beck, and Jim Sebastian from Colorado State University, initiated a study on reseeding Russian knapweed infested sites. The purpose of the study was to examine the effects of tillage and herbicide applications on Russian knapweed control and grass establishment. Specific objectives of the study were to 1) evaluate Russian knapweed control with various herbicides, 2) evaluate the impact of herbicide treatments on grass establishment, and 3) determine the effect of tillage on Russian knapweed control and grass establishment.


“We selected a large, uniform population of Russian knapweed for our study site. The site was previously farmed, but was recently converted to open space by Commerce City, Colorado,” explained Vassios. The site was located in central Colorado about 15 miles east of Denver. Individual plot size was about 20 by 70 feet arranged as a split-split plot design with four replications of each treatment. 

The study included three tillage treatments for seedbed preparation: no tillage, minimum tillage (coulter wheel), and full seedbed preparation (till and pack) as main plot treatments. Russian knapweed control treatments included a non-treated control, hand-weeded control, Tordon® 22K herbicide at 1.5 quarts of product per acre (qt/A), and Milestone® herbicide at 7 fluid ounces of product per acre (fl oz/A). Two grass species were selected as restoration species: slender wheatgrass (Elymus trachycaulus) and western wheatgrass (Pascopyrum smithii). 

The site was mowed to remove above ground vegetative growth in late October and tillage treatments were applied three days after mowing. Herbicides were applied in November in 20 gallons total solution per acre. Grass species were seeded in March, about four months following herbicide application. 

Visual injury ratings for rosettes and bolting plants were taken in July, 9 and 21 months after treatment, respectively. Above ground production (biomass) was collected for grass species and Russian knapweed one year after application. Hand-weeded controls were maintained periodically.


Results of the study showed Milestone and Tordon 22K provided greater than 99% Russian knapweed control 9 and 21 months after treatment (MAT) (Figure 1) (Photo). In contrast, the hand-weeded control only showed 85 and 66% control at 9 and 21 MAT, respectively.


In plots receiving physical or herbicide control treatments, seedbed preparation had no impact on Russian knapweed shoot density or biomass (Figure 2, Figure 3). However, in non-treated plots both minimum- and full-tillage treatments significantly reduced Russian knapweed biomass compared to non-tilled (Figure 3).

Herbicide treatments did not have a negative impact on seeded grasses (Photo). Western and slender wheatgrass biomass was lowest in non-treated and hand-weeded controls, and greatest in herbicide treatments (Figure 4). Milestone treated plots resulted in significantly greater grass biomass than either Tordon 22K or the hand-weeded control. Removing above-ground Russian knapweed by hand resulted in grass production that was not significantly different from control plots. 


Vassios says, “Results showing significantly less grass production in hand-weeded plots (Russian knapweed roots in tact) provide strong evidence that Russian knapweed allelopathy is a function of belowground interactions and is not a function of leachates from surface residues. We hope to conduct additional studies in the lab and field to examine possible allelopathy from intact Russian knapweed roots.”

Editor’s Note

Information in this article was presented in part at the Western Society of Weed Science Meeting in Hawaii in 2010 by Joseph D. Vassios, Scott J. Nissen, James R. Sebastian, and K. George Beck, Colorado State University, Ft. Collins, CO.

Extensive herbicide field studies have been conducted on Russian knapweed throughout the western United States. Results of these studies and optimum time to control Russian knapweed are summarized here.

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Milestone herbicide is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. Consult the label before purchase or use for full details.

Tordon 22K herbicide is a federally Restricted Use Pesticide. State restrictions on the sale and use of Transline herbicide apply. Consult the label before purchase or use for full details. Always read and follow label directions.©2019 Corteva

Published September 2011; reviewed and updated June, 2019.