Tumbling weeds

摘要

This is what an alien invasion looks like. Giant balls of tumbleweed are causing problems across the western US, where prolonged drought and high winds have led to a boom in their numbers. The most common tumbleweed, Kalitragus. Isn't native to the US - it is an invader from Eurasia, and is also known as Russian thistle. The alien hitched a ride in seed shipments to South Dakota in the 19th century, just in time to spread west and become a handy metaphor in cowboy movies for the desolation of ghost towns on the frontier. The plant grows in the normal way until it is mature and dry, when it breaks from its roots and rolls away, scattering seeds as it does so. This is a fantastic method of dispersal in an arid environment.