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Why Do Tumbleweeds Tumble? | Deep Look

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Channel: Deep Look
Categories: Environmental   |   Physics   |   Science  
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The silent star of classic Westerns is a plant on a mission. It starts out green and full of life. It even grows flowers. But to reproduce effectively it needs to turn into a rolling brown skeleton.

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Tumbleweeds might be the iconic props of classic Westerns. But in real life, they’re not only a noxious weed, but one that moves around. Pushed by gusts of wind, they can overwhelm entire neighborhoods, as happened recently in Victorville, California, or become a threat for drivers and an expensive nuisance for farmers.

“They tumble across highways and can cause accidents,” said Mike Pitcairn, who tracks tumbleweeds at the California Department of Food and Agriculture in Sacramento. “They pile up against fences and homes.”

And tumbleweeds aren’t even originally from the West.

Genetic tests have shown that California’s most common tumbleweed, known as Russian thistle, likely came from Ukraine, said retired plant population biologist Debra Ayres, who studied tumbleweeds at the University of California, Davis.

A U.S. Department of Agriculture employee, L. H. Dewey, wrote in 1893 that Russian thistle had arrived in the U.S. through South Dakota in flaxseed imported from Europe in the 1870s.

“It has been known in Russia many years,” Dewey wrote, “and has quite as bad a reputation in the wheat regions there as it has in the Dakotas.” This is where the name Russian thistle originates, said Ayres, although tumbleweeds aren’t thistles.

The weed spread quickly through the United States — on rail cars, through contamination of agricultural seeds and by tumbling.
“They tumble to disperse the seeds,” said Ayres, “and thereby reduce competition.”
A rolling tumbleweed spreads out tens of thousands of seeds so that they all get plenty of sunlight and space.

Tumbleweeds grow well in barren places like vacant lots or the side of the road, where they can tumble unobstructed and there’s no grass, which their seedlings can’t compete with.

--- Where does a tumbleweed come from?

Tumbleweeds start out attached to the soil. Seedlings, which look like blades of grass, sprout at the end of winter. By summer, Russian thistle plants take on their round shape and grow flowers. Inside each flower, a fruit with a seed develops.
Other plants attract animals with tasty fruits, and get them to carry away their seeds and disperse them when they poop.
Tumbleweeds developed a different evolutionary strategy. Starting in late fall, they dry out and die, their seeds nestled between prickly leaves. Gusts of wind easily break dead tumbleweeds from their roots and they roll away, spreading their seeds as they go.

--- How big do tumbleweeds grow?

Mike Pitcairn, of the California Department of Food and Agriculture, said that in the state’s San Joaquin Valley they can grow to be more than 6 feet tall.

--- Are tumbleweeds dangerous?

Yes. They can cause traffic accidents or be a fire hazard if they pile up near buildings.

---+ Read the entire article on KQED Science:

---+ For more information on the history and biology of Russian thistle, here’s a paper by Debra Ayres and colleagues:

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