There’s a biological reason; it’s not just for ambiance in cowboy Westerns.
Tumbleweeds are familiar to anyone who’s a fan of Westerns, the classic cowboy movies. A grizzled man strolls through the desert, spurs clinking on his boots, eyes squinting from beneath the wide brim of a dusty hat. From atop his horse, he gazes out at the barren desert, watching as a barren bundle of branches goes rolling along, propelled by the wind.
This isn’t just a cinematic choice for setting mood. Tumbleweeds are real, although they aren’t one specific species of plant; there are a bunch of different plants that we label as tumbleweeds.
Even the part that’s tumbling will vary from plant to plant. For some plants, it’s almost the whole thing, while it’s a specialized body in others.
But why do these plants tumble? Is this just what happens to old plants in the desert; they dry out and blow away?
Or is a tumbleweed actually a schemer, making use of the environment for a specific purpose?
Let’s look at what these plants are, and why they are far more active than most of their other vegetative brethren.
All the plants that form the tumbleweeds we love
Tumbleweeds are in a bunch of families of plants, including:
- Amaranthaceae (the family that includes chard, spinach, and beets)
- Brassicaceae (the cabbage family)
- Caryophyllaceae (the carnation family)
- and others.
Tumbleweeds don’t all derive from a common lineage; but appear to be an example of convergent evolution, where a similar feature has evolved independently, multiple times, in unrelated organisms. (Another example of convergent evolution: bats and whales both use echolocation, a form of high-pitched sound to locate features in their environment. Bats and whales do not share a common ancestor that used echolocation; they each evolved the ability independently to each other.)
So why do tumbleweeds tumble?
It’s the same reason why most organisms do anything; it offers a better chance of reproducing! Specifically, tumbleweeds have learned to make use of the strong winds in their regions to help spread their seeds.
After the tumbleweed plant has produced seeds, the entire plant above the root system dies, drying out and growing structurally weaker. This way, the wind can break away the connection between the shrub and the roots, letting it come away and roll.
In most tumbleweeds, the whole plant breaks off — but some species only shed a cluster of flowers, known as an inflorescence, and others will shed light, dried-out fruits that are also able to roll in the wind. Whatever is shed, however, carries the seeds of the plant.
Some tumbleweeds dispense their seeds as they roll, like a salt shaker leaving grains behind as it rolls across a table. Others will hold on to the seeds until the rolling portion of the plant comes to rest in a place with water. Often, contact with water will make the dried-out plant swell and break open, so that the seeds can land in a location where water exists.
Get out of here, tumbleweed, you’re not wanted
Tumbleweeds seem like an iconic part of the American Southwest, thanks to being commonly shown in movies and related media. But did you know that they’re actually an invasive species?
Most of the common North American tumbleweeds are members of the Amaranthaceae family; it’s believed that this group of tumbleweeds was introduced in the late 1800s, likely in or around South Dakota. From there, it spread throughout much of North America, taking over places with high winds, regions with sparse vegetation, areas with little rainfall, and along the sides of roads.
Overall, tumbleweeds are commonly viewed as a weed, a pest, and a crop to be eliminated when possible. They’re responsible for a number of issues, including:
- Spreading fires — the dry, brittle tumbleweed easily catches fire, and can spread that fire as it’s blown by the wind.
- Causing erosion — tumbleweeds drain large amounts of water from the soil when they grow, weakening it.
- Getting into unwanted places — because they are transported by the wind, tumbleweeds often end up in areas where humans don’t want them, like electrical facilities or water treatment plants.
- Causing blockages and pile-ups — there have been multiple instances of people literally being trapped in houses thanks to huge drifts of piled-up tumbleweeds! Towns have been buried with tumbleweeds multiple meters deep, requiring emergency services to free occupants trapped in their homes.
The U.S. and Mexican governments both have tumbleweed abatement programs, designed to try and drive out this invasive pest. Despite their efforts, however, tumbleweeds have become fairly endemic and aren’t likely to be driven back out of North America any time soon.
Currently, the focus involves mowing of fields where tumbleweeds are growing, cutting down the shrubs before they reach maturity and can detach and start tumbling. The USDA has also explored releasing parasites that specifically prey on tumbleweeds, but tests have not proven successful for this method.
In summary: tumbleweeds are seed dispensers, surrounded by a dead husk
Tumbleweeds are a class of plants that have all evolved to make use of the wind to help spread their seeds to new areas, preferably to areas with available water. Different species of plants will detach different portions of their structure, but the end result is a lightweight mass of dead plant matter, carrying seeds that will germinate and grow in a new destination.
Tumbleweeds are widely considered to be a pest; they’re an invasive species in North America, they can both fuel and spread wildfires, and when they are able to grow unchecked, they both out-compete local plants and can lead to major headaches for humans. Large clumps of stuck-together tumbleweeds can cause roadblocks, or even bury entire neighborhoods!
If you see a tumbleweed, or you have tumbleweeds growing in your yard, the best thing to do is to dispose of them. Do your part to help fight invasive species, and leave the tumbling dry husks to appear only in movies!
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