Armed with pointed tips so sharp that neither cows nor deer will eat it, medusahead (Taeniatherum caput-medusae) is an invasive grass species that seems to have stepped right out of the Little Shop of Horrors. With no enemies, it is spreading rapidly throughout the western United States, outcompeting native grasses and even other grass invaders. Unless steps are found to control its spread, medusahead is likely to turn millions of hectares of grazing land into worthless fields, say researchers in a study that determined why this grass is so successful.