How do Keep Off The Grass signs get there?

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...

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

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

People with REALLY long arms ;-)

By the art of levitation