Cenchrus longispinus

Cenchrus longispinus is a species of grass, also known as spiny burr grass or gentle Annie. Its fruits are clumped into "burrs" with sharp, barbed spines that can penetrate the hides and mouth of grazing animals. They can also become lodged in human clothing, causing some discomfort.
The species is native to North America (Canada, Mexico, and the United States). It is a noxious weed in Europe, Australia and New Zealand where it was introduced.