Botanists find one of ‘the world’s worst weeds’ spreading in the Boise foothills

Cogongrass is the latest of a fast-growing number of invasive plants threatening Idaho’s fire-prone rangelands.


In early May, invasive plant specialists in Idaho received a surprising and ominous report: Retired botanist Barbara Ertter had spotted a small but spreading patch of what the U.S. Department of Agriculture calls one of “the world’s worst invasive weeds” growing in Boise. She sent the details to a second local botanist, Ann DeBolt, who confirmed: Cogongrass, a perennial grass native to parts of East Africa and Asia, was growing along the roadside in a foothills subdivision.

A patch of cogongrass seeding in Boise, Idaho. The invasive grass is very difficult and expensive to eradicate.
Courtesy of Adam Schroeder/Ada County Weed, Pest and Mosquito Abatement

Its presence there was “extremely alarming,” DeBolt wrote to state experts. It was the first known report of the species in Idaho. DeBolt knew the grass well from her work in Florida, where it lines roadsides and has invaded mining sites, pastures and natural areas, displacing native species and boosting the speed and severity of wildfires.

Idaho is already struggling to manage invasive grasses, which increase the risk of wildfire in the Great Basin’s warming, drought-stricken rangelands. The sudden appearance of cogongrass in Boise also exemplifies how a warming climate allows species to thrive in places where their presence was once thought unlikely.

Cogongrass is a well-known problem in the Southeast, where it has infested more than 1.25 million acres, according to the USDA. Though the patch in Boise covers only about a tenth of an acre right now, this plant’s roots can spread 6 to 8 feet per year. Fire-adapted and extremely flammable, it spreads even faster after a blaze. Grazing animals won’t eat the grass, and it’s very difficult and expensive to eradicate. It spreads underground through rhizomes, and over land by foot traffic, vehicles and wind that pick up both root fragments and seeds from the plant’s fluffy white seed heads.

It’s not clear how cogongrass got to this particular Boise neighborhood, said Adam Schroeder, director of the weed and pest abatement program for Ada County, where Boise is located. He thinks it’s possible that it hitched a ride with someone who moved to Boise, one of the fastest-growing cities in the nation, or that a cogongrass variant had been planted nearby on purpose: A subspecies, sold under the names Red Baron or Japanese bloodgrass, is typically sterile but can revert to the original, fertile type and spread.

The plant is just one of a growing list of newcomers: Lately, Schroeder said, as a result of human migration and a warming climate, his office has to respond to reports of a new weed species just about every year. “When you think about what that looks like over the next 10 to 15 years, that’s a lot more work,” he said.

For at least 150 years, non-native grasses — including cheatgrass, medusahead, and ventenata — introduced by human migration, trade, and agriculture have dramatically altered the Boise and Great Basin landscapes. “That tends to be a one-way transition,” said Rob Bennett, a natural resource manager at the Bureau of Land Management’s Boise District.

These annual grasses rapidly seed and reseed disturbed landscapes, preventing native perennial grasses and sage from taking root. They also fuel a worsening cycle of larger, hotter wildland fires that tear faster and more frequently across the rangelands, burning through sagebrush habitat and ranches and opening up yet more areas for invasive grasses to spread further. After a fire, their seeds can germinate in just a few weeks, easily outcompeting native plants, unless land managers quickly apply pre-emptive, selective herbicides and work hard over time to both slow their spread and replant native perennials and sage.

Homes in the foothills surrounding Boise, one of the fastest-growing cities in the nation. It’s likely that a cogongrass seed was transported by someone that moved to the area, or a subvariant of the grass was used in a home landscaping project.
Gary O. Grimm/CC via Flickr

Cogongrass could add to this already challenging situation. It tolerates a wide range of conditions, including drought, and thrives along roadsides where the risk of wildfire ignition is high. Once established, cogongrass is very difficult to control: The only treatments known to work are glyphosate and imazapyr, broad-spectrum herbicides that kill most everything else, too. “If you have to wipe out the entire area, that makes things much more difficult to rehabilitate,” said Caleb Ashby, who leads post-fire emergency stabilization and rehab efforts for the BLM’s Boise District. “There are things worse than cheatgrass and medusa out there.”

Schroeder has already started treating Boise’s small patch of cogongrass. To be effective, he said he’ll need to apply herbicide across the site every six weeks or so for the next two to four years. “I’m a pretty optimistic guy, so I’m 100% confident that we’re going to knock this thing out everywhere we find it,” he said. But he’s less certain that it hasn’t already spread elsewhere, or that weed managers will be able to find all of it — steep local terrain and the potential for home landscape plantings to revert to fertile grass makes it difficult to survey.

“There are things worse than cheatgrass and medusa out there.”

Another unknown is climate change: The plant’s behavior in tropical regions like the Gulf Coast is well understood, but it might behave differently in places like Idaho, where rising temperatures could be aiding its spread. Experts had long assumed that colder temperatures would prevent it from getting a foothold in northern regions, but that hasn’t stopped the patch in Boise, which Schroeder believes could be about 5 years old.

In the weeks after DeBolt’s initial report, the Idaho Department of Agriculture temporarily added cogongrass and its subspecies, Japanese bloodgrass, to the state noxious weed list. For at least 15 months, all sales of the plants will be forbidden, while agencies work with landowners and enforce eradication efforts statewide.

Schroeder advises Boise residents to notify his office if Japanese bloodgrass or Red Baron grass has been planted in their yards so the department and its partner agencies can map the locations and help develop plans for removal. One such planting has already been identified by a homeowner who purchased the plant at a local nursery.

Before DeBolt’s email warning about the grass’s presence in the foothills, “I might have driven right past this infestation,” Schroeder said. He hopes the attention raised by listing it as a noxious weed in Idaho will result in quick and effective action. “Every instance of this plant needs to be eradicated,” he said, “before it becomes something that is well out of our control.”

The Institute for Journalism & Natural Resources supported reporting for this story.

Sarah Trent is an editorial intern for High Country News based in southwest Washington. Email her at [email protected] or submit a letter to the editor. See our letters to the editor policy.

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