New species of California tumbleweed rolls into town, weighing 13 pounds, standing 6 feet tall

Tumbleweeds are an iconic sight in Southern California deserts: brownish dead plants rolling harmlessly across seemingly barren landscapes.

But there's a new kid in town and that kid is big. Really big.

The relatively new hybrid tumbleweed is Salsola ryanii. It's twice the size of its parents, can weigh almost 13 pounds, can reach 6 feet tall — and as an invasive species it's here to stay.

Those are the vital statistics as determined in a study by University of California, Riverside professor Norman Ellstrand and Chapman University post-doctoral fellow Shana Welles.

Aside from its size, the new weed looks just like but dwarfs its nearly identical parent species, Salsola australis and Salsola tragus (commonly known as Russian thistle). Salsola tragus originated in Central Asia and made it to America in the 1800s, while Salsola australis, native to South America and Australia, arrived in the early 1900s.

Like a match made in invasive species heaven, the two met in America and subsequently produced their hybrid tumbleweed child, Salsola ryanii.

The new species was first discovered by experts in California in 2002. They realized the tumbleweed was the product of hybridization, but predicted that it would soon go extinct because they had only uncovered three small populations of it.

Several years later, Welles and Ellstrand started studying the offspring for Welles' Ph.D. dissertation. For her first paper, she focused on its spread by traversing the state to hunt for tumbleweeds. Based on her findings, she and Ellstrand determined that it had lived up to its reputation as an invasive species, making its way from where it was first discovered, in Chico and Bakersfield, all the way to the UC Riverside campus.

Welles' second paper was a more in-depth genetic analysis of the hybrid, and her third study, published in July, shows that plant reproduction has a tendency to create species that are more than just new — they're also more vigorous. This is the result of a process known as allopolyploidy, in which species are created with more than two sets of chromosomes. This comes with benefits, like the ability to tolerate environmental extremes, but renders the new hybrid species sterile.

Ideally, the vigor of a hybrid species like Salsola ryanii would be determined by its seed production and measured by growing the hybrid and its parents together. To avoid contributing to the tumbleweed apocalypse, however, Welles used the Agricultural Operations space at UC Riverside to grow the species to maturity, but cutting them off before they could spread their seeds. She found that the hybrid weighed about two times as much as its parents and determined its increased seed production from there.

"Depending on where they're growing, their size can be really variable," Welles told The Desert Sun, noting the role of environmental conditions in tumbleweed growth. "All of them can get like 5.5 feet tall, but if they're growing somewhere that's not as good, they can also be 1 foot tall and flower that way."

Because of this variation, and the plant's dependence on water during its growing season, not even the experts know how climate change will affect the growth and spread of the hybrid.

Based on how it changes, though, locals might have some reason to worry when it comes to driving.

"Tumbleweeds are bad to begin with," Ellstrand said. "This is going to be somewhat nastier. How nasty depends on both its genetics and also its environment, but it will be the tumbleweed that's annoying you more frequently."

Kaylee Beam is a The Desert Sun intern.