How do Keep Off The Grass signs get there?

The Keep Of The Grass sign fairy plants them at night.

A helicopter hovers down a person tied to a rope is lowered down to stick it in. This is a hard and dangerous job that requires 2 years of training. If they fail, and touch the grass, or the rope breaks, the person is shot. Then another hovering helipcopter must come in and lower a person down to clean it up...

By the art of levitation

People with REALLY long arms ;-)

They were put there by the long arm of the law, smartass