There are few images as iconic to the American West as a lonely tumbleweed rolling through a dusty street in some desolate one-horse town.
In reality, many cities and towns are working hard to relegate this symbol of the West to the pages of history as well.
Tumbleweeds encompass several types of invasive species that can damage native ecosystems, farms, and property. Now, according to researchers, a much larger hybrid species could prove even harder to control with current measures like spraying herbicides or mowing.
“Salsola ryanii is a nasty species replacing other nasty species of tumbleweed in the U.S.,” said Norman Ellstrand, a professor of genetics at the University of California, Riverside, and co-author of a recent study.
Salsola ryanii is a cross between the two most abundant tumbleweed varieties, Salsola australis and Salsola tragus, and can grow up to six feet tall, much larger than its parent species.
As the hybrid expands its range, it could intensify the annoying or even menacing effects of its progenitors, like fouling up oil and gas pads, reducing farm yields, and even covering residents’ yards.
“They are going to be larger, more likely to be prolific,” Ellstrand said. “An individual tumbleweed can easily create 5,000 seeds.”
Tumbleweeds are among the most efficient seed distributors in the plant world. While rooted to the soil, they flower and grow. But when mature, their sphere-like shape allows them to break free and roll with the wind across the landscape, dislodging hundreds of seeds with each bounce.
Farmers detest tumbleweed because it competes with dry-land crops, such as wheat, for water and nutrients, and it can reduce yields.
In addition to the headaches for agriculture, tumbling tumbleweeds are also a hazard to traffic and have been known to spread forest fire.
In 2014, tumbleweeds basically buried the town of Clovis, N.M., requiring the U.S. Air Force to help dig the residents out.
A similar situation happened in April 2018, when high winds stacked tumbleweeds up to the eaves of houses in the desert town of Victorville, Calif.
“It’s also a really big issue on oil and gas pads,” said Aaron Easley, a weed-control specialist at Colorado-based Horizon Environmental Services.
Over the years, Easley says, his company has eliminated tumbleweeds from industrial rights of way and oil and gas pads in the Four Corners region of Colorado, Utah, New Mexico, and Arizona.
The trick, Easley says, is to remove or kill the plants while they are still seedlings: “They are a summer annual, so in May they are still small plants, but by August they can be really big.”
Applying common herbicides such as dicamba or glyphosate usually kills tumbleweeds, he said, if applied before the plants have dried up and gone to seed.
In addition to the threats posed by more robust hybrid weeds, there are also reports that tumbleweed, like other weeds, may be developing a resistance to glyphosate, the world’s most common herbicide.
Agricultural researchers say the prospect of resistance underscores the need to diversify weed control strategies to preserve the viability of herbicides in semi‐arid crop systems.
“This is not good news for growers,” said Judit Barroso, a weed scientist at Oregon State University.
In a blog post for Oregon State, Barroso said Russian thistle, the most common type of tumbleweed, is already costing American farmers $50 million annually in control measures.
The Agriculture Department has been looking into deploying plant pathogens to control tumbleweeds.
“In my mind, the trend for controlling invasive plants is turning against chemical controls, and we have already discovered a potential solution that is free and effective,” said Dana Berner, a retired USDA plant pathologist, who spent several years looking at the potential of fungus to control tumbleweed.
The approach Berner was researching involved exposing tumbleweed populations to fungal spores from the plant’s original Eurasian territory, which would cause widespread disease in the weeds.
“We explored this fungal method on field tests in Greece and Russia, and it completely eliminated the weeds,” he said. “It was an excellent biocontrol agent.”
In only two seasons, Berner said, the fungal pathogen completely eliminated sites that were 100% infested with Russian thistle, which he says would likely work on the hybrid, Salsola ryanii, as well.
While the pathogens attacked the tumbleweed, he said, it wasn’t observed to spread to other plant species.
In order to release an exotic agent domestically, it would need approval by a technical committee of the USDA’s Animal and Plant Health Inspection Service.
“This particular advisory group appears to be extremely paranoid,” Berner told Bloomberg Environment. “But the results we observed were very encouraging. I think the risks are small compared to the benefits we could see by eliminating this dangerous and costly species.”
Tumbleweeds are said to have been accidentally imported to the U.S. in a shipment of flax seed in the late 1800s.
The hybrid tumbleweed was first discovered in California’s Central Valley in 2002. It has since expanded much faster than previously thought, in part because it is well-adapted to environmental changes being exacerbated by climate change, Ellstrand, the UC-Riverside scientist, said.
“It’s one of the only things that’s still green in late summer,” said Shana Welles, a plant biologist at Chapman University and the lead author of the study.
“They may be well positioned to take advantage of summer rains if climate changes make those more prevalent,” Welles told UC-Riverside.
Welles also found that unlike most hybrid species, which can’t reproduce, Salsola ryanii has a special chromosomal structure that not only allows it reproduce, but “do so more vigorously than either of its parents.”
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