Once mushroom spores blow into your lawn they need a couple of things to grow into the mushrooms. First, there needs to be plenty of moisture available…mushrooms love dampness and won’t grow in dry lawns. In a damp year like last year mushrooms seemed to spring up everywhere. In a normal season the lawn usually dries out well between rainstorms so mushrooms are less of a problem. Very often mushrooms are a problem in a lawn that has a sprinkling system installed. Folks with these sprinkling systems seem to like to see them operate and have them set to water the lawn frequently but not very heavily. This causes all kinds of problems for the lawn including encouraging mushrooms and, worse yet, fungal lawn diseases. Properly programmed, a sprinkling system should provide 1″ of water per week in one single watering. This amount of water will penetrate at least 8″ into the soil stimulating deep root growth. It also allows the blades of the grass many day of dryness which makes it much harder for mushrooms and fungal diseases to take hold. Everyone with a sprinkling system should do a test to see how long it takes each zone to put out 1″ of water. This can be done by placing a small tuna fish or cat food can in the zone and run it until the can is full and note how long it took. Then each zone should be set to run for that amount off time just once a week. The second requirement for mushroom growth is the presence of rotting organic matter. This could be an old tree stump rotting below the surface, buried construction debris or something as simple as rotting grass clippings and leaf debris. Removing buried debris and bagging your clipping can help prevent mushroom growth. Mushrooms don’t harm the grass and are actually helping the lawn by breaking down organic matter that the lawn will eventually benefit from. The main reason to eliminate mushrooms is for the safety of small children who might eat them.