Tumbleweed invasion has its roots in fire, drought
For the past two weeks, the area around Colorado Springs has been besieged by tumbleweeds. Piles of the dessicated, rolling shrubs have clogged streets and yards, trapping residents at home in at least one case. For farmers, the plants have choked off irrigation ditches, the piles of tinder-dry shrubs forming fire hazards.
Clearly, Colorado is in the midst of a tumbleweed boom, with an unusual number of the noxious weeds popping up in the central and eastern parts of the state. And this likely won't be the last season that we spend buried in the brush.
"We are in the midst of a drought, and that is going to give the Salsola tumbleweed a competitive advantage," says Greg Cronin, an associate professor in the University of Colorado Denver's integrative biology department. "And with all the fires we had a couple years ago, that certainly is going to create habitat where Russian thistle will thrive."
While many different kinds of plants can form tumbleweeds, most of Colorado's crop comes from Russian thistle, or Salsola tragus, a spiny, fruiting shrub common throughout the state. After the annual plant dies, it dries up and detaches from its stem, scattering hundreds of thousands of seeds as it blows in the wind.
Despite tumbleweeds' status as a symbol of the frontier, Russian thistle is actually invasive: The plant arrived on American shores in the nineteenth century, likely in a shipment of flaxseed.
A hardy immigrant, the plant quickly found a niche in the United States. Russian thistle is what is known as a "pioneer species," a plant that thrives in disturbed areas where competition is scarce, and the dry, harsh climate and occasionally poor soils of the American West suited it just fine. By the early twentieth century, it had propagated throughout the region, taking root in roadside ditches and parched pastures; according to the USDA, it nowgrows in every state except for Florida and Alaska
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The shrub's love of disturbed environments is probably why Colorado Springs is reaping such a bumper crop of tumbleweeds this year, Cronin says. Over the past two years, two record-setting fires scorched over 32,500 acres around the city, creating prime habitat for Russian thistle. The recent spate of high winds was the trigger that the dried-up plants needed to start rolling into town.
This year's invasion might be just the beginning. If, as many forecasters predict, climate change leads to longer droughts and more frequent blazes, more Coloradans could find themselves busting out their snow shovels to clear tumbleweeds out of their driveways in years to come.
"Only time will tell," says Cronin. "But I guess what I might hypothesize is that if the drought continues, and we continue to have these tremendous fires that disturb a lot of communities, that does set up a situation where Russian thistle should do quite well.
That's not to say that a boom in Russian thistle couldn't have its silver lining. In its younger, greener form, the protein-rich plant can be used as forage for cattle, and is a potential source of biomass for sustainable fuels, as well as citrates, used in food preservation and other industrial applications. Cronin and several other researchers from CU Denver even investigated the feasibility of domesticating it. (Russian thistle has to be harvested young for most uses, so a domestic crop wouldn't add to the tumbleweed problem.)
For now, Coloradans in tumbleweed-stricken areas have no choice but to wait it out. Some neighborhoods have turned to plows to clear vital routes, while local government and business leaders in the eastern part of the state have formed a task force to come up with a strategy for dealing with the weeds. As for whether this is the start of an annual tradition, we'll have to wait and see.
Follow Adam Roy on Twitter: @adnroy
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