The true tumbleweeds are various species of herbaceous plants in the amaranth family (Amaranthaceae). These are usually annual plants that develop a spherical, bush-shaped biomass. At the end of the growing season when their small seeds are ripe, the tumbleweeds wither and detach from their base and are blown about by winds, scattering their seeds widely over the surface of the ground. Therefore, the tumbling habit of these plants is an adaptation to extensive dispersal of their ripe seeds.
One common species of tumbleweed is Amaranthus graecizans. This annual plant is native to semi-deserts but is now a common weed of recently disturbed land in agricultural and urban areas. This plant has a whitish stem, green leaves, and numerous small, greenish flowers. Amaranthus albus, also known as tumble pigweed, is a closely related tumbleweed.
Other unrelated species of plants also tend to tumble to disperse their seeds. Two examples in the goosefoot family (Chenopodiaceae) are the Russian-thistle or Russian tumbleweed (Salsola kali) and the Russian pigweed (Axyris amaranthoides). These are both introduced species and can be important weeds. The winged pigweed or tumbleweed (Cycloloma atriplicifolium) is a related native species of arid habitats of the West which has also become weedy in open, disturbed habitats. Another species with a tumbling habit is the tumble mustard (Sisymbrium altissimum), in the mustard family (Brassicaceae). Tumble panic-grass (Panicum capillare, family Poaceae) produces a large, bushy inflorescence that often detaches and blows about at the end of the growing season.
The "tumbling tumbleweeds" have become a romanticized element of the landscape of the American West through the pervasive influence of their images in songs and movies. However, many of these very capable and opportunistic tumbleweeds are also extremely widespread as weeds. As such, the tumbleweeds are taking advantage of many types of disturbed habitats that are created on the landscape by humans and their activities.