The tumbleweed that rolls down the empty street at the start of a western is most often Russian thistle, and it is not from the West. It was first recorded in North America in Bon Homme County, in what was then Dakota Territory, in the 1870s, its seeds most likely mixed into flaxseed brought over from the Russian Empire and sown by farmers who only wanted flax. (One historian is sure it was already in Kansas by then; either way, it came in a sack.) When the plant dries in autumn it snaps off at the base and rolls, shaking out seed as it goes, and within a few decades it was all over the West. So when the films needed a plant to say this place is empty and nobody has settled here, the one they chose had come in with the settlers, hidden in what they meant to plant.