They plant a seed where they want the sign to grow when the lawn is planted.
Simple. People who are that caring about their grass are obviously rich. And what do all rich people have? Hovercars! The Woodburninator
They bribe local children to go put up the sign. As soon as the sign goes in, the child is trespassing and is lawfully shot in the eye. This feeds the local animals for the next few weeks, until the remains rot into the ground and serve as fertiliser for the plants that grow there. And when the animals start to get hungry... "Hey little kid, go check that sign's in the ground nice and proper......." *bang*