One year’s seeds, seven years’ weeds

Herbicide treatments for flowering rush in Lake Pend Oreille

By Soncirey Mitchell
Reader Staff

Lake Pend Oreille’s noxious weed problem has worsened over the past six years, as blooms of invasive Butomus umbellatus, or flowering rush, have joined Eurasian milfoil to clog area waterways. The U.S. Army Corps of Engineers has been unable to treat invasive aquatic weeds since 2017, due first to lack of funds and then complications caused by the COVID-19 pandemic.

That changed Aug. 8-14, when the Corps enacted targeted herbicide treatments at portions of Riley Creek, Morton Slough, Oden Bay East, the Pack River and Clark Fork River deltas, and the Clark Fork Drift Yard. Flourishing yet manageable patches of the weed were identified in each of these locations.

“Applying herbicides on these colonies will control most of the flowering rush, limiting its spread to unaffected areas and preventing dense monoculture stand development,” the Corps wrote in a statement to the Reader.

Flowering rush seen near Riley Creek. Photo courtesy U.S. Army Corps of Engineers.

According to the Corps, the densely packed flowering rush can “degrade water quality (including impacts on irrigation and potable water), decrease aquatic biodiversity, negatively impact fish and wildlife habitat (including bull trout), harbor swimmers itch and interfere with shoreline activities.” 

Swimming, fishing and boating are just some of the activities threatened by the invasive plant, which mainly reproduces by cloning itself from small bits of growth, called rhizomes. Fragments of flowering rush can be transported on many kinds of recreation equipment, including fishing gear, boots and all manner of watercraft. Without proper cleaning, the rhizomes can spread from location to location, where they quickly establish themselves in thick patches.

As is common with noxious weeds, flowering rush is difficult and tedious to remove.

“Hand digging and removal of rhizomes and buds after drawdown has occurred in various locations around the lake with some success,” Idaho Conservation League North Idaho Director Brad Smith told the Reader in an email, “but care must be taken to contain and remove all plant fragments from the water because the dispersal of rhizomes can contribute to further spread.”

Smith said flowering rush was likely introduced to Lake Pend Oreille from Flathead Lake in Montana, and the Corps anticipates that Lake Pend Oreille’s native plant population will regrow once the flowering rush is eradicated, preventing its future growth.

As there is not one specific herbicide proven to work best on flowering rush, the Corps uses EPA-approved diquat dibromide (a.k.a. Reward), diquat dibromide and endothall (a.k.a. Aquastrike) or glyphosate (a.k.a. Rodeo), depending on the site. The treatments are different from the herbicide ProcellaCOR used to eradicate the Eurasian milfoil population.

Herbicides are used both in conjunction with — and as an alternative to — physical control methods; however, they have the potential to harm native plant species growing in or near the application sites. Additionally, the leftover decomposing plant matter draws oxygen from the water, which can harm nearby fish.

“From our perspective, there is a need to strike a balance between treating aquatic invasive species and the introduction of potentially harmful herbicides,” added Smith.

There were no fishing or swimming restrictions in place at the treatment sites, however, locals were advised to wait three days before drinking the water or irrigating their landscapes, and five days before watering their crops, according to the Corps’ treatment notice.

“The extensive toxicology studies required for aquatic use labels clearly indicate that there are no dermal (skin) uptake issues with swimmers, and fish metabolize and depurate the products very quickly, so no uptake problem with them either,” the Corps stated.

The drinking water restrictions are an added safety precaution, though all the herbicide treatment plots were at least 600 feet from any potable water or irrigation sources.

The treatment notices were posted only 48 hours in advance — according to the Idaho Conservation League, that isn’t adequate.

“Notification and awareness associated with treatments is of critical importance because different individuals have different sensitivities to exposure,” wrote Smith. “The most critical failure on the part of the Army Corps and the Idaho State Department of Agriculture was their failure to adequately notify the public of their plans.”

Going forward, the Corps hopes to use the herbicides imazapyr and imazamox. The products would be applied to the flowering rush in early spring while the lakebed is exposed. 

“No issues with water quality or bull trout would occur because there will be no water present during treatments,” according to the Corps. “After water returns to the treated areas, and native plants start to sprout, the product has already degraded, and native plants grow unharmed.”


*CLARIFICATION (Aug. 25, 10:47 a.m.): The Reader received an additional email from the U.S. Army Corps of Engineers on Aug. 24, further explaining its use of glyphosate. “We want to clarify that we are not treating any submersed flowering rush with glyphosate, as that is not an approved product for any submersed plant,” the Corps stated.

Officials also responded to the Idaho Conservation League’s statement that treatments were applied without enough prior notification: “We reached out to notify all the municipalities … on July 25, 2023, and signs were posted at all nearby boat ramps on the weekend of July 21, 2023. … We did face vandalism and high winds which knocked a few of our signs down; however, they were all resurrected as soon as possible, and they were posted more than 14 days prior to treatment per the requirement. We also posted additional signage on shorelines where treatments were taking place two days before treatments occurred. Furthermore, we notified the Beacon, Sandpoint Daily Bee and the Newport Miner on July 19, 2023 with [a] news release. Those entities chose not to run the article.”

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