Fall Application Timing Controls Russian Knapweed in Wyoming Watershed

THE LITTLE POWDER RIVER AND COTTONWOOD CREEK ARE IMPORTANT TRIBUTARIES in the Powder River Basin of northeastern Wyoming. Together they drain more than 1,200 square miles before flowing into the Powder River, and ultimately the Yellowstone River in southeastern Montana.

Russian knapweed (Acroptilon repens) is well established in both drainages, and there is growing concern among area ranchers and the weed and pest district about rapid spread of the weed.

“In the summer of 2010 we were working with landowners in the watershed to control a grasshopper outbreak,” explains Quade Schmelzle, supervisor of Campbell County Weed and Pest District. “The effectiveness of the grasshopper program helped solidify the credibility of the weed and pest district and generated interest in initiating a control program on Russian knapweed.”

Since the majority of concerned landowners were in the upper reaches of the drainage, the weed and pest district initially focused efforts in that area. “Our goal was to start the control program on the uppermost infestations and then gain support from downstream producers,” says Schmelzle. 

The first step was to inventory the area to determine the size of the infestation and develop a management strategy. Each summer for three years a four-person crew recorded the location of Russian knapweed infestations within the 50-mile area. Location waypoints were buffered by 100 feet to account for smaller plants that may have been missed during the survey. By the end of 2014, crews had documented about 5,000 acres of Russian knapweed within the project area (Map 1).

Cottonwood trees are scattered throughout the treatment area and Russian knapweed grows in grass meadows and under the canopy of many of the trees. Schmelzle explains, “We didn’t want to damage the cottonwoods but we needed to control the knapweed. In 2012 we brought in specialists with Dow AgroSciences and Van Diest Supply to help with recommendations. They suggested a fall aerial application of Milestone® herbicide at 7 fluid ounces per acre (fl oz/A) when cottonwood trees were dormant.” 

The project area was divided into three phases, beginning at the top of the watershed and working downstream. The first phase of the project began in the fall of 2013 when 1,600 infested acres were treated. 

Results nine months after treatment were excellent with about 90 percent control of Russian knapweed and no damage to cottonwood trees. “The success we had with the first phase of the project increased landowner participation downstream to nearly 100 percent,” says Schmelzle. In 2014 an additional 2,600 acres were treated with similar results, and about 2,340 acres are scheduled for treatment in fall of 2015. 

Although Campbell County Weed and Pest District paid the entire cost of the application plus 80 percent of the herbicide cost, landowners are responsible for follow-up maintenance. “We have signed agreements with all 17 cooperators in the project area since continued maintenance is critical to the long-term success of this project,” says Schmelzle. 

The strength of the project is based on accurate inventories, careful planning, and trust between landowners and the weed and pest district. “The best of intentions and high aspirations aren’t enough if you lack cooperation and a good working relationship with the landowners. It is very important to be organized, patient, and sure that everyone agrees on project goals and objectives. It’s a time consuming process to get everything in place, but the extra effort pays off,” concludes Schmelzle.


Related Articles: 

Managing Russian knapweed on western rangelands with Milestone herbicide

Protecting Watershed Values in Southeast Wyoming

Establishing Desirable Grasses in Russian Knapweed Infested Sites

Published 2015; Trademark language updated June 2019

®™Trademark of Dow AgroSciences, DuPont or Pioneer and their affiliated companies or respective owners. Milestone 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. See the product label for details. State restrictions on the sale and use of Milestone apply. Consult the label before purchase or use for full details. Always read and follow label directions.©2019 Corteva