Source: Wikipedia.

A tumbleweed is a structural part of the above-ground anatomy of a number of species of plants. It is a diaspore that, once mature and dry, detaches from its root or stem and rolls due to the force of 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 inflorescence might detach instead. Xerophyte tumbleweed species occur most commonly in steppe and arid ecosystems, where frequent wind and the open environment permit rolling without prohibitive obstruction.

Apart from its primary vascular system and roots, 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 its seeds or spores can escape during the tumbling, or germinate after the tumbleweed has come to rest in a moist location. In the latter case, many species of tumbleweed open mechanically, releasing their seeds as they swell when they absorb water.

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Below is a recent video from CNN of tumbleweeds in a Utah neighborhood.