This past Saturday I was briefly but genuinely concerned that my dog Tycho would be injured by tumbleweeds. From a distance, they seem almost cotton ball–like: airy and round, perfectly suited to catching a breeze and bouncing along their merry way. Up close, though, they reveal themselves as spiny plant carcasses, sometimes several feet in diameter, stiff enough to scratch up cars and lacerate skin. When the wind picks up and whips them at you in 60 mph ambushes, watch out.

The term “tumbleweed” refers to a whole slew of plant species that go all out to spread their offspring each year. So what makes these iconic invaders unique and enables them to take over the Southwest each year?