A tumbleweed is the above-ground part of any of a number of plants that, once mature and dry, disengage from the root and tumble away in the wind. Usually, the tumbleweed is the entire plant apart from the roots, but in a few species it is a flower cluster. The tumbleweed habit is most common in steppe and desert climates. The tumbleweed is a diaspore, aiding in dispersal of propagules (seeds or spores). It does this by scattering the propagules either as it tumbles, or after it has come to rest in a wet location. In the latter case, the tumbleweed opens mechanically as it absorbs water; apart from its propagules, the tumbleweed is dead.
Plants forming tumbleweeds
Although the number of species with the tumbleweed habit is small, quite a number of these species are common agricultural weeds.
Although thought to be native to Eurasia, several annual species of the genus Kali (formerly Salsola, both in family Amaranthaceae) that form tumbleweeds have become so common in North America that they are a common symbol in Western movies, where they are typically symbolic of desolation in frontier areas. Kali tragus (Syn. Salsola tragus or Russian thistle) is an annual plant that breaks off at the stem base, forming a tumbleweed that disperses its seeds as it rolls on top of the ground. It became naturalized over large areas of North America after being imported from continental Asia often in shipments with agricultural seeds. Kali tragus is said to have arrived in the United States in shipments of flax seeds to South Dakota in the nineteenth century (perhaps around 1870-1874). It has become a noxious weed that has spread throughout North America to inhabit suitable habitats which include areas with disturbed soils like roadsides, cultivated fields and eroded slopes, and in natural habitats that have sparse vegetation like coastal and riparian sands, semi-deserts and deserts. K. tragus is an extremely variable species with many races which vary in distinctness. Some of these varieties in the past have been divided into subspecies or even separate species. Though it is a noxious weed, K. tragus is useful on arid rangelands as forage for livestock.
Other members of the family Amaranthaceae that form tumbleweeds include Amaranthus albus, native to Central America but introduced and weedy in Europe, Asia, and Australia; Amaranthus graecizans naturalized to North America from its native Africa; Amaranthus retroflexus; Corispermum hyssopifolium; Kochia; and Cycloloma atriplicifolium, which is called the plains tumbleweed.
In the aster family (Asteraceae), Centaurea diffusa (a knapweed) forms tumbleweeds. This species is native to Eurasia and naturalized in much of North America. Also in this family, Lessingia glandulifera sometimes forms tumbleweeds; it grows in desert areas, chaparral, and open pine forests of the western United States and is usually found on sandy soils.
In the legume family (Fabaceae), species reported to produce tumbleweeds include some members of the genus Psoralea, and Baptisia tinctoria. In some species, the tumbleweed is detached from the plant by abscission of the plant stem; abscission has been shown in Psoralea and Kochia.
The garden plant "baby's-breath" (Gypsophila paniculata), which is in the pink family Caryophyllaceae, has 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.
Diaspores made of inflorescences occur also 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. 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.
In Southern Africa genera of Amaryllidaceae such as Boophane, Crossyne, and to a lesser extent Brunsvigia, bear inflorescences in the form of globular umbels with long, spoke-like pedicels, either effectively at ground level, or breaking off once the stems are dry and the seeds ripe. The light, open, globular structures form very effective tumbleweed diaspores, dropping the 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 strategy for such plants.
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, and Physaria.
Very similar in habit to Anastatica, but very distantly related, 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 use essentially the same dispersal strategy.
Tumbleweeds have a significant effect on wind soil erosion in open regions, particularly on dry-land agricultural operations where the outside application of additional moisture is impossible. One study showed that a single Russian Thistle can remove up to 44 gallons of water from the soil while competing with a wheat crop. The amount of water removed from fallow land more subject to erosion would be even higher. In addition to the moisture consumed by the plant, significant damage to the protective soil crust is caused by the tumbleweeds' motion. The damage to the soil surface then provides exposure for subsequent wind damage and topsoil loss.
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 comedic effect, in locations where tumbleweeds are not expected. (One example is in the opening credits 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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