An iconic symbol of the West, tumbleweeds conjure images of cowboys, cattle drives, and barbed wire. They even have their own song—“Tumbling Tumbleweeds” was written by Bob Nolan in the 1930s, and seems to reappear as often as the weeds for which it’s named.

Yes, we’re all familiar with tumbleweeds. But, as a result of my Colorado Master Gardener training, I have insider information that will stun, shock, and astonish you. Tumbleweeds are aliens!

Yes, it’s true. Before the Europeans stumbled across the western hemisphere, there were no tumbleweeds on the plains. Of course, there were no cowboys, either—no horses, no cattle, and no chuckwagon bean dinners. Tumbleweeds arrived, not in flying saucers, but in seed shipments from Europe and Asia.

There are several plant species that dry up, break from their roots, and turn into tumbleweeds. Around the end of the 19^th century, seeds of the genus Salsola (the most widespread of the tumbling plants) arrived as stowaways in bags of flax and other seeds intended for agriculture. It doesn’t take much moisture to germinate a tumbleweed seed, and soon the invaders had spread throughout the arid west.

Amazingly, once the USDA recognized how well adapted the species was, it actively introduced S. tragus across the west in hopes of providing a drought-resistant forage plant for livestock. (Then, upon realizing how incredibly ill-advised this decision was, our government eventually did a complete about-face, and now lists this plant as a noxious weed.)

Salsola iberica (synonyms: S. kali, S. pestifer) is another related plant that forms tumbleweeds. It arrived from Russia in the late 1800s, hence its common name, Russian Thistle.

Plants in the genus Salsola can now be found from the scorched Death Valley desert to altitudes reaching 8,500 feet in the Colorado Rockies. They thrive in dry agricultural fields, on overgrazed rangeland, and can even be found growing on irrigated cropland.

The whole point of a tumbleweed is to spread its seeds as widely as possible. My parents found this out the hard way. Newcomers to California, they struggled to recreate their childhood Christmas memories without the snow and cold of the northeast. One year they got the bright idea to haul some tumbleweeds home from the vacant field at the end of our street. Spray-painted white, stacked, and adorned with hats and scarves, their tumbleweed “snowmen” stood proud in our front yard. Cars slowed as the drivers gawked at the innovative decorations.

After the holidays, the snowmen were bagged, set out with the trash, and forgotten—until spring. That’s when a million baby tumbleweeds sprouted amid the dichondra. (Dichondra is a walk-on groundcover that forms a no-mow lawn substitute in warm climates. It’s generally low maintenance, but because it isn’t grass, you can’t use selective chemical weed killers on it.) My memories of that spring and summer are mostly of my parents on their knees, digging out these tenacious, deep-rooted weeds.

Vast sums have been spent in an effort to eradicate these environmental intruders. It takes repeated applications of the strongest herbicides to bring them to their knees. But consider—one tumbleweed contains approximately 250,000 seeds. Now add in the open landscape and unrelenting winds of the plains, perfect for a plant that spreads by rolling across the ground. Unlike the cattle drives of the Old West, it’s pretty obvious that tumbleweeds are here to stay. They join the long list of other alien species that have made our country their home.