Giant Hybrid Tumbleweed Spreads in California

Now that's a tumbleweed. A truck bed-sized specimen collected by Shana R. Welles in Riverside, California. . (Photo: University of California, Riverside)

California has enough to worry about on the invasive species front, thank you very much, but it's now also home to a monster hybrid tumbleweed.

Researchers at the University of California, Riverside, have discovered that two large invasive species of tumbleweed have joined together to create a new invasive species of tumbleweed, and it's spreading quickly. They recently took a closer look at this large tumbleweed to see why it was thriving.

They found that the species, Salsola ryanii, is much larger than either of its parent plants, which can grow up to 6 feet tall. Findings from their study published in AoB Plants show that the tumbleweed likely grows faster and stronger because it has two pairs of its of its parents' chromosomes.

UC Riverside genetics professor and study co-author Norman Ellstrand described Salsoa ryanni as "nasty" in a university news release.. "It's healthier than earlier versions, and now we know why," he said.

Climate change could also help extend its territory. Even though the tumbleweed is an annual, it begins to grows in late winter and stays green until late summer, taking full advantage of summer rains.

Finding the tumbleweed

The monster tumbleweed was first documented by scientists with the California Department of Food and Agriculture back in 2002 in two areas of the state's Central Valley. The plant, a hybridization of Salsola australis and Salsola tragus, was found to have spread to a third area of the Central Valley in 2009.

In a earlier news release published by UC Riverside, it's noted that invasive Salsola australis — as its name would suggest — is native to Australia (or maybe South Africa) and can be found throughout California and Arizona.

Also known as prickly Russian thistle, Salsola tragus is the classic, clichéd tumbleweed largely associated with podunk outposts of the American West — that is, this large and in-charge introduced species of Eurasian origin is so commonplace that's come to be generically known simply as "tumbleweed." (Keep in mind that the term tumbleweed technically refers to the aboveground anatomic structure belonging to a variety of plants that break off from their roots after they become dry and mature, dispersing seeds as they roll away in the wind.)

As UC Riverside explains, the "problematic" Salsola tragus, believed to have the most rapid spread of any introduced species, is found in 48 states. It first arrived on American shores in the late 19th century aboard contaminated shipments of flax seed from Russia.

'The potential to be a problematic invasive'

But back to the unholy union of these two tumbleweeds. In 2012, researchers from UC Riverside found that Salsola ryanii had indeed spread well beyond the confines of the Central Valley to the San Francisco Bay Area and Ventura County, northwest of Los Angeles, in just over a decade.

In a paper published by the American Journal of Botany, Shana R. Welles, a former UC Riverside graduate student working alongside advisor Ellstrand, officially declared Salsola ryanii an invasive species, noting that it has the potential to spread from California to neighboring states and beyond.

"Given how quickly it has spread, this species has the potential to be a problematic invasive," explains Welles. "We want to make sure people know that and try to manage this species when it still had a relatively narrow range."

Welles notes that average folks won't be able to differentiate Salsola ryanii from the two species of tumbleweed that spawned it. The hybridized plant was identified via DNA testing of plant samples collected at 53 different sites. Of these sites, Salsola ryanii was present at 15 — or 28% — of them, contradicting earlier notions that it would not spread beyond isolated areas of the Central Valley.

While Welles concludes her findings in a most ominous manner — "We are not aware of any plant neospecies whose range spontaneously experienced such a dramatic expansion. Salsola ryanii has every indication of being just as invasive as its highly invasive parents" — at least she and her colleagues have a sense of humor about it, as displayed in the above video depicting the not-so-elaborate mating ritual of two lonely tumbleweeds looking for a little sweet lovin'.