When we think of a desolate plain or a foreboding frontier town in the wild west, we might think of the iconic tumbleweed rolling through the scene. Or they might roll through as a joke. But for people who live in dry parts of western North America, the tumbleweed is, in fact, a weed that can block doors or clog waterways as they gather in piles. They’re neighborhood nuisances that create fire hazards. They also cause accidents when they roll out onto roadways.
As it turns out, tumbleweeds are not native to the United States. They’re invasive Russian thistles that flower, die, dry up into a spiny skeletal ball, and roll. Why? Seed dispersal. From KQED via PBS NewsHour:
Why the Giant Sequoia Needs Fire to Grow, exploding plants disperse their seeds with high pressure bursts, and TED Ed’s Symbiosis and a surprising tale of species cooperation.
Tumbleweeds start out as any plant, attached to the soil. Seedlings, which look like blades of grass with a bright pink stem, sprout at the end of the winter.
By summer, Russian thistle plants take on their round shape and grow white, yellow or pink flowers between thorny leaves. Inside each flower, a fruit with a single seed develops.
Starting in late fall, they dry out and die, their seeds nestled between prickly dried leaves. Gusts of wind easily break dead tumbleweeds from their roots. A microscopic layer of cells at the base of the plant — called the abscission layer — makes a clean break possible and the plants roll away, spreading their seeds. When the rains come, an embryo coiled up inside each seed sprouts.