Salsola tragus, often known by its synonym Kali tragus, is a species of flowering plant in the family Amaranthaceae. It is known by various common names such as prickly Russian thistle, windwitch, or common saltwort. It is widely known simply as tumbleweed because, in many regions of the United States, it is the most common and most conspicuous plant species that produces tumbleweeds. Informally, it may be known as Kali or Salsola: the latter being its restored genus, containing 54 other species, into which the obsolete genus Kali has been subsumed.

For a brief phase during its youth, it may be grazed but afterward becomes too spiny and woody to be edible to most wildlife and livestock (if it is not processed first). Mature specimens are often more than a meter in diameter. As its fruits mature, the diaspore of the plant dies, dries, hardens, and detaches from its root. This detached anatomical part of Salsola tragus is colloquially called "tumbleweed" (although there are many other plant species that also produce tumbleweeds). Once mature, dry, and detached from the plant, this tumbleweed will tumble (i.e., roll) due to the force of the wind. As this dead structure tumbles in the wind, it gradually degrades and falls apart, thereby spreading possibly as many as 200,000 seeds. If it happens to come to rest in a wet area then it can germinate rapidly, even with very small amounts of moisture. It has a high tolerance of salinity and can successfully compete with many native plants in certain environments, such as along sea beaches and especially in grassland, desert, or semiarid regions. Consequently, it now occupies a wide variety of habitats.

Native to Eurasia, Salsola tragus has proven to be highly invasive as an introduced species and rapidly became a common ruderal weed of disturbed habitats throughout the world. The tumbleweed's tumbling is known to damage non-native plants and environments and its highly flammable nature also sometimes helps wildfires spread, especially during windy conditions. An ignited tumbleweed may spread a fire across firebreaks and may even ignite buildings or structures that it stops against.

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