Mushrooms are the fruiting bodies of fungi in the soil. Typically mushrooms are a result of some kind of decomposing wood in the soil – old tree roots are a common cause of mushrooms. You most likely have some mushrooms in your lawn. There is not much you can do about them except for pull them or knock them over as they arise. Don’t eat them though, you never know what mushrooms are ok to eat and what mushrooms could be a problem.