An Invasive Tumbleweed Is Taking Over California

The hybrid weed’s speedy expansion threatens native plants and animals along with agriculture, a new study finds.
The Salsola ryanii tumbleweed, a hybrid of tenacious invasive weeds from Africa, Russia, and Asia, is expanding across California. (Photo: Shana Welles/UC Riverside)
Apr 8, 2016· 1 MIN READ
Katharine Gammon has written for Nature, Wired, Discover, and Popular Science. A new mom, she lives in Santa Monica.

An exotic weed tumbling across California threatens to upend native ecosystems and complicate the lives of farmers, according to a new report.

This tumbleweed, a hybrid of invasive plants hailing from Australia, Africa, and Eurasia, was first described in 2002, when researchers identified it in California’s Central Valley.

When they went back to assess its movements a decade later, they found it had extended its range across a 400-mile swath from San Francisco to Ventura.

“Even compared with other invasive species, this expansion is really rapid,” said Shana Welles, a plant scientist at the University of Arizona and the lead author of the new study, published in the journal American Journal of Botany.

One of the hybrid tumbleweed’s parents, Salsola tragus, “is considered a problematic weed in 48 U.S. states,” according to the study, “and has been described as having the most rapid spread of any introduced species.”

Welles and a colleague, Norman Ellstrand, found the hybrid tumbleweed, dubbed Salsola ryanii, in 15 of 53 sites sampled up and down California, compared with just three sites in 2002.

Tumbleweeds tumble to disperse their seeds in a way that’s not unlike a dandelion lofting its fuzz into the wind: When the plants are rooted to the ground, they flower, and then when they disconnect from the ground, they send out seeds to make more tumbleweeds. “The seeds aren’t viable until the plant is dry and tumbling,” said Welles.

The hybrid species has the potential to cause problems for farmers, because like other weeds, it competes with crops for nutrients and water. It has also become a road menace as it tumbles onto California freeways.

Because it is a hybrid of two invasives, Salsola ryanii has no native range, leaving scientists with nowhere to hunt for insects that might control its spread. That leaves pulling the plant by hand or using herbicides, said Welles. “When the plant is small, rabbits will eat them,” she said. “But when it’s bigger, not a lot can eat it, and efforts at bio-control haven’t been very successful.”

She and Ellstrand concluded that the tumbleweed has the potential to infect other states or countries, given its origins and super-plant expansion. “It definitely needs to be managed in a thoughtful way,” she said.

Other types of exotic tumbleweed, such as Russian thistle, are outcompeting native plants for food and water and leaving little for foraging wildlife to munch on. The Nature Conservancy, a nonprofit conservation group, has resorted to tractors, sling blades, and even GPS tracking to try to manage the invasive thistle.