Why Do Tumbleweeds Tumble? | Deep Look

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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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DEEP LOOK is a ultra-HD (4K) short video series created by KQED San Francisco and presented by PBS Digital Studios. See the unseen at the very edge of our visible world. Explore big scientific mysteries by going incredibly small.

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 really 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.”

By bouncing and rolling, a 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 abandoned agricultural fields, 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 as any plant, attached to the soil. Seedlings, which look like blades of grass with a bright pink stem, sprout at the end of the winter. By summer, Russian thistle plants take on their round shape and grow flowers. Inside each flower, a fruit with a single 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 they can grow to be more than 6 feet tall in parts of the state like the San Joaquin Valley.

— Are tumbleweeds dangerous?

Yes. They can cause traffic accidents, and they can be a fire hazard if they pile up against buildings.

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