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Irrigation-clogging weed arrives in Oregon

Flowering rush, a Eurasian aquatic weed, was found for the first time in Oregon this summer and is already creating problems in other states by clogging irrigation canals.
Mateusz Perkowski

Capital Press

Published on October 3, 2014 9:57AM

Flowering rush, an invasive Eurasian aquatic weed, was recently discovered for the first time along the Columbia river in Oregon. The weed poses a danger to agriculture because it can block irrigation canals and irrigation intakes.

Photo by Meenerke Bloem from Wikimedia Commons

Flowering rush, an invasive Eurasian aquatic weed, was recently discovered for the first time along the Columbia river in Oregon. The weed poses a danger to agriculture because it can block irrigation canals and irrigation intakes.


An aquatic weed capable of clogging irrigation canals and other waterways has appeared along the shores of the Columbia River in Oregon for the first time.

Six patches of flowering rush, an invasive Eurasian species, up to 10 feet in diameter were found growing east of McNary Dam near Umatilla, Ore., this summer.

However, the weeds have yet to be removed as state officials must confer with the U.S. Army Corps of Engineers, which has jurisdiction over the river, about federal permits for eliminating the plants.

“It’s still sitting there, waiting,” said Dan Hilburn, administrator of plant programs at the Oregon Department of Agriculture, noting that state officials have a meeting with the federal agency on Oct. 23.

Hilburn hopes the Army Corps of Engineers is able to streamline the permitting process so the weeds can be dug up before they spread further downriver.

“It just takes over,” he said. “What used to be open water turns into solid flowering rush.”

The Army Corps of Engineers did not respond to a request from Capital Press to explain what permits may be involved.

Flowering rush is a threat to agriculture primarily because it forms thick masses of vegetation that impede the flow of water, Hilburn said. “The whole function of the irrigation system is to move water and this one would really hamper that.”

It’s fortunate that the patches were discovered before they’d grown larger, as the weed is difficult to treat with herbicides and must often be removed manually, said Tim Butler, supervisor of ODA’s noxious weed control program.

“We want to find them when they’re very small infestations,” he said.

Apart from obstructing irrigation canals, flowering rush can get sucked into irrigation intakes and block screens and pumps, said Peter Rice, research ecologist at the University of Montana.

Montana has several large established populations of the weed, and flowering rush has also created problems for irrigators in Idaho, he said.

The Aberdeen-Springfield Canal Co. in the eastern part of the state spent five years excavating its irrigation system with expensive aquatic vegetation rakes to be rid of the plant, Rice said.

“It gets so thick it restricts the water delivery,” he said.

Because flowering rush colonizes previously open water and dramatically alters the habitat, it’s considered an “ecosystem engineer,” said Rice.

Its propensity to change ecosystems is of particular interest in the Pacific Northwest because it poses a hazard for threatened fish, he said.

The fate of threatened salmon and steelhead are of concern to farmers and ranchers who may face restrictions on irrigation and grazing.

In autumn, the stems and leaves of the perennial plant drop to the river bottom but do not decay, providing a place for the northern pike — an introduced species that preys on native fish — to attach its eggs.

The pike’s eggs would fall into the substrate and suffocate in open water, but the eggs stick to the flowering rush debris. When the weed emerges in spring, its leaves and stalks shelter the young pike.

Killing the weed with herbicides is tough because few chemicals are labeled to be sprayed in or near water, and those that can be applied this way often aren’t effective, said Hilburn.

Using herbicides in flowing water is also challenging because some chemicals require lengthy contact time with the plant but get washed off in streams and rivers, said Jenifer Parsons, aquatic plant specialist with the Washington State Department of Ecology.

“So far there has not been a good control method identified,” she said. “It’s a really tough plant to kill.”

Washington has several populations of flowering rush, including one on the Yakima river that is the likely source of the weeds in Oregon and new sites on the Columbia river in Washington, Parsons said.

The weed doesn’t produce many seeds but it spreads with brittle buds that break off from its rhizomatous roots, she said. Those buds are prone to falling off whenever the soil is disturbed.

“Even with hand digging, you have to go back several times to control the population,” said Parsons.



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