The Whitehall Ledger - Serving Southern Jefferson County in the Great State of Montana

By Jack H. Smith
Ledger Publisher 

Herbicide results released

Jefferson Slough


November 23, 2016

Last July, a herbicide was placed in the Jefferson Slough in an attempt to knockout Eurasian watermilfoil prior to a planned relocation of the Slough.

According to Jefferson County Weed Coordinator Jill Allen, Dr. Ryan Thum of Montana State University and students from the School’s Plant Science and Plant Pathology Department did the monitoring of the application. Dr. Thum recently released the team’s results.

In the report, Dr. Thum said they sampled the slough for pre-treatment and abundance on both July 10 and 11.

Dr. Thum stated they quantified abundance using a rake toss index at each of the 100 sampling points established along the Jefferson Slough using GPS.

“The rake toss index is as follows: 0 for no watermilfoil, 1 for 1 living watermilfoil plant, 2 for less than 25 percent of the rake tine space covered in watermilfoil, 3 for 25-50 percent of the tine space covered in watermilfoil, and 4 for greater than 50 percent of the tine space covered in watermilfoil,” Dr. Thum said.

According to Dr. Thum, the team also conducted genetic identifications on one to three plants from each location with watermilfoil to estimate the composition of watermilfoil at each point.

“There was a very clear pattern of pure Eurasian watermilfoil in upstream portions of the Slough and hybrid watermilfoil in the downstream portions of the slough, with a mixture of pure and hybrid watermilfoil in some points in between. This was the same pattern we expected based on previous sampling of the Slough in 2014 and 2015,” Dr. Thum stated in the report.

Thum said they repeated the sampling post-treatment in last August, approximately six weeks after the treatment.

“The general observation from post-treatment sampling was that there was clearly a lot of die back of watermilfoil. There were some spots where the watermilfoil was completely gone and other spots where there were just black stems remaining,” Dr. Thum said.

He did however state that in some spots, there were plants that had obvious symptoms of injury, but meristems that looked they were “growing through” the injury; and in some cases, even flowering.

“Additionally, there were a handful of spots where the watermilfoil looked like it was minimally injured. These spots were mostly (if not exclusively) in riffles with shallow and relatively fast-flowing water. For these reasons, it will be important to re-sample the watermilfoil in early summer, 2017, to determine the extent of vegetative regrowth of both pure and hybrid Eurasian watermilfoil in the slough. It should be noted that endothall is a contact herbicide, which means that it injures/kills tissues that it comes into direct contact with, but is not translocated throughout the plant to kill all tissue,” he reported. “So, some vegetative regrowth would not be surprising. Of specific interest to us is whether the extent or rate of vegetative growth is higher for either pure or hybrid Eurasian watermilfoils, as the rate at which they re-grow has implications for how frequently applications might be needed to keep watermilfoil abundance to acceptable levels.”

The herbicide used in the application was Cascade, a product of UPI, based out of King Prussia, PA. UPI employee Craig Smith told the Ledger in July that 90 to 95 percent control would be a reasonable expectation. The funding for the herbicide application comes from money obtained from the 2013 session of the Montana Legislature.

Ted Dodge, Watershed Coordinator for the Jefferson River Watershed Council, said in July they would need to knockout the milfoil before they start work on the Jefferson Slough channel relocation.

Dodge said construction on the relocation is expected to begin in fall or winter.

According to the JRWC 2015 Annual Report, the rapidly spreading Eurasian Milfoil out competes native plans and impacts irrigation and hydropower facilities, recreational activities, and wildlife habitat.

Dodge said the milfoil was first discovered in 2011, right at the time they were starting to address flooding and also sediment issues in Big Pipestone Creek.

“You could make a case the sediment coming down the slough was a perfect nursery for the milfoil,” Dodge said.

Allen added they want to get the milfoil knocked back so it lessons the risk of cross contamination in the new stream bank. The annual report also states that it is estimated that the Jefferson Slough contributes about 17,000 pounds of Eurasian Milfoil fragments annually to the Jefferson River, and these fragments are establishing new infestations in sections of the Jefferson, Madison and Missouri Rivers.


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