A tumbleweed is a structural part of the above-ground anatomy of any of a number of species of plants, a diaspore that, once it is mature and dry, detaches from its root or stem, and tumbles away in the wind. In most such species the tumbleweed is in effect the entire plant apart from the root system, but in other plants a hollow fruit or an inflorescence might serve the function. Tumbleweed species occur most commonly in steppe and arid ecologies, where frequent wind and the open environment permit rolling without prohibitive obstruction.
Apart from its propagules, the tissues of the tumbleweed structure are dead; their death is functional because it is necessary for the structure to degrade gradually and fall apart so that the propagules can escape during the tumbling, or germinate after the tumbleweed has come to rest in a wet location. In the latter case, many species of tumbleweed open mechanically, releasing their seeds as they swell when they absorb water. 
The tumbleweed diaspore disperses propagules, but the tumbleweed strategy is not limited to the seed plants; some species of spore-bearing Cryptogams such as Selaginella form tumbleweeds, and some fungi that resemble puffballs dry out, break free of their attachments and are similarly tumbled by the wind, dispersing spores as they go.
Tumbleweed migration is essential to the survival of the species. A group of tumbleweeds traveling together is commonly referred to as a flock, and when crossing roads can be deterred by honking the car horn. Tumbleweeds often travel long distances and it can be noted their current distance traveled by the roundness of the plant.
Plants that form tumbleweeds
The tumbleweed dispersal strategies are unusual among plants; most species disperse their seeds by other mechanisms. Many tumbleweeds (though by no means all) are ruderal species, opportunistic agricultural weeds. Tumbleweeds have been recorded in the following plant families:
- Amaranthaceae (now including Chenopodiaceae)
In the family Amaranthaceae (s.l., including Chenopodiaceae), several annual species of the genus Salsola are the most notorious tumbleweeds. They are thought to be native to Eurasia, but when their seeds entered North America in shipments of agricultural seeds, they became naturalized in large areas. They have been so successful that in the cinema genre of Westerns they have long been symbols of frontier areas. Salsola tragus (currently not valid synonym Kali tragus) is the so-called "Russian thistle". It is an annual plant that breaks off at the stem base when it dies, and forms a tumbleweed, dispersing its seeds as the wind rolls it along. It is said to have arrived in the United States in shipments of flax seeds to South Dakota, perhaps about 1870. It now is a noxious weed throughout North America, dominating disturbed habitats such as roadsides, cultivated fields, eroded slopes, and arid regions with sparse vegetation. Though it is a troublesome weed, Salsola tragus also provides useful livestock forage on arid rangelands.
Other members of the Amaranthaceae (s.l.) that form tumbleweeds include Kochia species, Cycloloma atriplicifolium, and Corispermum hyssopifolium, which are called plains tumbleweed. Atriplex rosea is called the tumbling oracle or tumbling orach.
Among the Amaranthaceae (s.s.) that form tumbleweeds there are several species of Amaranthus, such as Amaranthus albus, native to Central America but invasive in Europe, Asia, and Australia; and Amaranthus graecizans, native to Africa, but naturalized in North America. Amaranthus retroflexus, which is indigenous to tropical North and South America, has become nearly cosmopolitan largely as a weed, but like many other species of Amaranthus it also is widely valued as animal forage and as human food, though it should be utilised with caution to avoid toxicity.
Several Southern African genera in the family Amaryllidaceae produce highly optimised tumbleweeds; their inflorescences are globular umbels with long, spoke-like pedicels, either effectively at ground level, or breaking off once the stems are dry. When the seeds are about ripe, the fruit remain attached to the peduncles, but the stem of the umbel detaches, permitting the globes to roll about in the wind. The light, open, globular structures form very effective tumbleweed diaspores, dropping their seeds usually within a few days as the follicles fail under the wear of rolling. The seeds are fleshy, short-lived, and germinate rapidly where they land. Being poisonous and distasteful, they are not attractive to candidate transport animals, so the rolling diaspore is a very effective dispersal strategy for such plants. Genera with this means of seed dispersal include Ammocharis, Boophone, Crossyne and Brunsvigia.
In the Asteraceae, the knapweed Centaurea diffusa forms tumbleweeds. It is native to Eurasia and is naturalized in much of North America. Also in the Asteraceae, Lessingia glandulifera, native to America, sometimes forms tumbleweeds; it grows on sandy soils in desert areas, chaparral, and open pine forests of the western United States.
In the Caryophyllaceae, the garden plant "baby's-breath" (Gypsophila paniculata), produces a dry inflorescence that forms tumbleweeds. In parts of central and western North America, it has become a common weed in many locations including hayfields and pastures.
Inflorescences that act as tumbling diaspores occur in some grasses, including Schedonnardus paniculatus and some species of Eragrostis and Aristida. In these plants, the inflorescences break off and tumble in the wind instead of the whole plant, much as happens in some of the Apiaceae and Amaryllidaceae. The species of Spinifex from Southeast Asia are prominent examples of this dispersal adaptation. These grasses are often called tumble-grasses, including such species as Panicum capillare and Eragrostis pectinacea in the United States.
Wind dispersed fruits that tumble or roll on the ground, sometimes known as "tumble fruits", are rare. Some are technically achenes. Highly inflated indehiscent fruits that may facilitate tumbling include Alyssopsis, Coluteocarpus, Physoptychis, Sutherlandia and Physaria.
Very similar in habit to Anastatica, but practically unrelated, are the spore-bearing Selaginella lepidophylla (a lycopod) and earthstar mushroom family (Geastraceae). All of these curl into a ball when dry and uncurl when moistened.
Bovista, a genus of puffball, uses essentially the same dispersal strategy.
Some ruderal species that disperse as tumbleweeds are serious weeds that significantly promote wind erosion in open regions. Their effects are particularly harmful to dry-land agricultural operations where the outside application of additional moisture is not practicable. One study showed that a single Russian Thistle can remove up to 167 liters (44 gallons) of water from the soil in competition with a wheat crop. The amount of water removed from fallow land more subject to erosion would be even more damaging.
It sometimes happens that species of large tumbleweed, especially if thorny, can form aggregations that are physically hazardous and can block roads and cover buildings and vehicles. This can most obviously happen where fences and similar obstacles cause the accumulation, but the weeds also can entangle each other spontaneously until they form piles that can no longer roll. Such a pile can be a serious threat to trapped vehicles or buildings and their occupants, most particularly because they are dry and flammable. Examples of enveloped buildings and vehicles have been documented mainly in the Western regions of the USA. One example was in the open on highway 349 in West Texas. In residential areas an example was in South Dakota in the town of Mobridge, where tens of tonnes of large tumbleweeds ("Russian thistles") that had matured in the dry bed of nearby Lake Oahe buried many houses so deeply that mechanical equipment was necessary to remove it, release occupants, and counter the fire hazard.
The tumbleweed's association with the Western film has led to a highly symbolic meaning in visual media. It has come to represent locations that are desolate, dry, and often humorless, with few or no occupants. A common use is when characters encounter a long abandoned or dismal-looking place: a tumbleweed will be seen rolling past, often accompanied by the sound of a dry, hollow wind. This is sometimes used, for comic effect, in locations where tumbleweeds are not expected. (One example is in the opening scene of the film The Big Lebowski.) Tumbleweeds can also be shown to punctuate a bad joke or a character otherwise making an absurd declaration, with the plant rolling past in the background and the wind effect emphasizing the awkward silence, similar to the sound of crickets.
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