A tumbleweed is a structural part of the above-ground anatomy 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 (that is, its seeds or spores), 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.
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.
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.
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