Monster Hybrid Tumbleweed Species Is Taking Over California, Scientists Warn

A new invasive species of tumbleweed that can grow up to six feet in height is taking over parts of California—and scientists are warning it could spread even further as climate change makes its growing conditions more favorable.

Salsola ryanii was first identified in California in 2002. It is a hybrid made up of two other invasive species—Salsola tragus, which is native to Russia and China, and Salsola australis, from Australia and South Africa. The latter, scientists say, is "one of the world's worst weeds" and is currently found in 48 U.S. states. The new species, is however, far bigger and faster growing than its parents, reaching about six feet in height.

A tumbleweed is a plant that breaks away from its roots towards the end of summer. It is blown around by the wind—its means of seed dispersal. In doing this, tumbleweeds cause huge problems. They can lead to traffic accidents and damage property. Invasive species also cause problems for the agriculture industry and native ecosystems.

In April 2018, the city Victorville in California made headlines across the globe after being buried in tumbleweed. City officials posted a photo of an affected house, saying high winds had caused the pile up.

After S. ryanii was first found, researchers started to document the plant's range.. They soon discovered it was spreading fast—in the last two decades its range has expanded to be several times larger than when it was initially identified. The tumbleweed is now found in California and Arizona.

In a study published in the journal AoB Plants, Shana Welles, from Chapman University, and Norman C Ellstrand, from University of California Riverside, looked at how and why S. ryanii has become so successful in such a short space of time.

"Salsola ryanii is a nasty species replacing other nasty species of tumbleweed in the U.S.," Ellstran said in a statement. "It's healthier than earlier versions, and now we know why."

Unlike animals, which tend to reproduce with one set of chromosomes from a mother and another from a father, plants can produce offspring with more than two sets. These plants are known as polyploids. Most of the time these offspring are unable to reproduce themselves—when a species is new it is at a disadvantage as it has nothing exactly like it to reproduce with.

Being a hybrid also has advantages, however. It is thought that polyploidy can result in a stronger, healthier plant. If it did not grow stronger than its parents, it would quickly go extinct.

S. ryanii, has double chromosomes from both parents. In their research, Welles and Ellstrand found it grew far more vigorously than either S. tragus or S. australis. They used two garden studies from 2012-2013 and 2014-2015, with two sites selected where all three species grow. Plants were grown from seed and distributed.

At the end of the season, they looked at the mass and seed numbers for hundreds of plants. Findings showed that S. ryanii was far more successful than its parents: "We document an increase in above-ground plant mass and above-ground volume in the newly formed allopolyploid compared to its progenitors," they wrote. The average mass of S. ryanii was 5.8kg, while its parents weighed an average of 3kg.

Concluding, the researchers say the new tumbleweed species is likely to expand its range even further over the coming years: "This fitness comparison combined with previous work documenting S. ryanii's ongoing rapid range expansion demonstrates that S. ryanii has the potential to become a problematic invasive species."

They also say the species may spread faster and further as a result of climate change. S. ryanii grows later than other species, so could benefit from increased rainfall predicted in California under some climate models. "They may be well positioned to take advantage of summer rains if climate changes make those more prevalent," Welles said in the statement.

Speaking to Newsweek, she added: "It is hard to know how climate change will impact future distributions of these species, but certainty as temperature and rainfall patterns shift it will impact the ranges of many species."

This article has been updated to include quotes from Shana Wells and more information about the weight of the tumbleweeds.