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Tumbleweeds are movie icons. We often see them rolling along in vast and desolate landscapes, acting as the supporting cast in scenes featuring Stetson-wearing cowboys.
While tumbleweeds may be no more than a prop in your favourite Western, they are the primary antagonist in the story of one of the fastest plant invasions in the history of the United States.
Tumbleweed, wind witch and Russian cactus are among the many common names for the Russian thistle (Salsola tragus) – though it isn't a thistle or a cactus at all and is instead part of the Amaranth family.
This plant is an icon of the Old West, featured in movies, song lyrics and more, however, its representation in media may be somewhat misleading.
Russian thistle is highly invasive in North America. It has followed in the wake of agriculture and other human activities since the late 1800s and its ability to spread prolifically makes it a threat to ecosystems, human health, and the economy.
Its invasion was also, at least in part, one of our own making and we're still living with the repercussions of it.
Russian thistle isn't native to North America. It is instead native to dry and semi-dry regions throughout Europe and central Asia.
The Russian thistle made its first known appearance in North America in the 1870s, in Bonhomme County, South Dakota. It's thought to have arrived surreptitiously; its inconspicuous seeds well hidden in a containment of flaxseed imported from what was then the Russian Empire.
Farmers were among the first to notice its arrival. Russian thistle is an expert at exploiting loose, disturbed soil with little competing vegetation. This is exactly what it found in the ploughed land of the Great Plains. As pioneer farmers cut down prairie grasses and other native vegetation to make space for crops, they created a suitable habitat for the invasive Russian thistle.
As Russian thistle matures, it goes from a soft seedling to a stiff and spiny plant. Its defences put off grazing animals, but also inflicted wounds on farmers, their horses and livestock. In some areas, infestations grew to the point that ploughing became impossible and there were extensive crop losses worth millions of dollars.
The plant also affected native vegetation and wildlife, preventing them from thriving in infested areas.
Russian thistle can spread prolifically and while it has its own very effective natural methods for doing so, its invasion of North America was at least in part facilitated by people.
By virtue of being about the same size, it was difficult to separate Russian thistle seeds from other cereal grains mechanically and threshing machines, which remove grain seeds from their stalks and husks, could inadvertently disperse the weed's seeds locally as they travelled between farms.
The seeds continued to contaminate consignments of grain. With the help of the transcontinental railroad, they were able to spread hundreds of miles with no effort on their part. The seeds and plants could also travel considerable distances by floating along in irrigation canals and ditches.
By the turn of the twentieth century, Russian thistle had made its way to California, where overgrazing had depleted sagebrush desert of it native plants, leaving ample new habitat to exploit.
The arrival and spread of Russian thistle is considered to be one of the fastest plant invasions in the history of the United States. Today the plant is found in all states except Alaska and Florida.
While the Russian thistle's rampant spread through North America is perhaps the more well-recorded invasion, this species has also been introduced to other parts of the world, including northern Europe and South America.
There are also some native species of tumbleweed in North America, such as Amaranthus albus, though these aren’t considered problem species.
Russian thistle is an exceptionally successful plant.
They produce a huge number of seeds – up to 250,000, depending on the size of the plant. Their strategy for dispersing them is also highly effective.
The plant grows in a rounded shape, sometimes to over a metre tall. By the onset of frost its seeds have matured, and the plant cleanly breaks away from its root. With a gust of wind, it begins to roll, becoming a true tumbleweed. Separated from its root, the plant is dead, but its thousands of seeds survive for months and as the plant bounces along, the vigorous tumbles shake them from their protected home among the spiny branches, leaving trails of them in the plant's wake.
When the seeds fall in an ideal location, they have to wait for favourable conditions to be able to germinate. Many plants require a complicated set of stimuli for germination to occur, such as the correct light, humidity and temperature conditions. The Russian thistle, however, only relies on temperature, needing a day temperature of about 20°C and 5°C at night to sprout.
These plants will grow in environments with extremely little moisture and are also adapted to alkaline and saline soils. This means they can grow in places with naturally reduced vegetation, as well as manually cleared land.
But a tumbleweed doesn't choose where to go - it travels wherever the wind pushes it. This means that it can get caught up on fence lines and other structures, or in established vegetation where it can't compete. Unless the wind changes and they are blown free, the plant's seeds will eventually die or be eaten by animals.
When Russian thistle arrived in North America around 150 years ago, it presented huge problems, particularly for farmers. It is less troublesome today, though it does still pose some threats.
For example, a field thick with Russian thistle can aid in the spread of wildfire. A tumbleweed's tangle of dry branches is particularly flammable and as the dead plants roll their way across fire lines or accumulate against structures like houses, they can become a threat to life should they catch alight.
Tumbleweeds can cause traffic accidents when they blindly bounce across highways, and herds of these spiny plants have been known to bury cars and houses. In 2014, two counties in Colorado declared a state of emergency when neighbourhoods became overwhelmed. In the previous year, 45 miles of roads had to be closed after being clogged by tumbleweeds.
They can block free-flowing water and pile up along fence lines and irrigation equipment causing extensive damage. Russian thistle can also be a host for insects that transmit curly top viruses, which stunt the growth of an infected plant and deforms its leaves and fruit.
The plant's pollen is a potent allergen, causing problems for people who suffer from hay fever or breathing issues and there are some reports that the sap can irritate skin.
Many are keen to control the prolific Russian thistle. Herbicides have been effective, however, they also negatively affect other plants and Russian thistle has been known to become resistant to some varieties.
A biological control method is a potential solution. This means using other organisms to restrict a pest species. But so far there isn't a viable option for Russian thistle.
Two species of moth were released as biocontrols in the 1970s, however they ultimately weren't effective. Investigations have been underway for the potential use of a blister mite or weevils found in Tunisia and Kazakhstan.
Russian thistle is a pest and it poses numerous problems for native flora and fauna and for human health. However, historically, people have become reliant on it at times, such as during the dust bowl in the 1930s.
With crops failing and few plants able to grow, the cattle industry and many people were left fighting for survival in the North American prairies.
Russian thistle provided a lifeline as one of the only plants able to grow despite the adverse conditions.
In its young, softer phase of growth, Russian thistle is palatable for herbivores, such as elk, bison, pronghorn and prairie dogs. During the dust bowl, farmers took to using the plant in place of traditional hay. For example, Kansas produced over 350,000 tonnes of thistle hay in 1934 alone. It's credited by some as having saved the cattle industry.
But it wasn't just cattle that survived on the plant. People reportedly turned to it to sustain themselves too. One family's experience, documented in the book The Worst Hard Time, was of surviving on a diet largely consisting of 'canned tumbleweeds'.
For a plant that is simply a background prop in many movies, Russian thistle took the main role in its own story of arriving and thriving in the waning days of the Wild West.
With human activities being one reason Russian thistle could so easily exploit North American habitats, this plant provides us with another clear lesson on how we impact our natural world.
The tumbleweed is still an iconic plant, but as the main antagonist in one of the fastest plant invasions the United States has ever seen, it may be so in more ways than you ever realised.