A scarecrow is an object made to resemble a human figure that is set up to scare birds away from a field where food is growing. A garden is a piece of land where vegetables or plants are grown. Garden sounds like "guardin'" or "guarding" which means to watch over in order to protect. This is a play on the word garden because it sounds like "guarding" and the joke is referring to a scarecrow protecting crops.
Scarecrows are always garden their patch .