Mushrooms reproduce with spores that spread throughout the yard, causing new mushrooms to sprout up in the grass. Killing mushrooms in your garden is a long-term process and you need to prepare yourself for the idea that you may never be able to permanently eradicate them. You'll either need to eliminate your garden or learn to live with the mushrooms and simply remove them when they appear.
Remove any item in your yard that provides a surface for the mushrooms to grow. Mushrooms grow in areas with a damp surface, including rotten trees and dead or dying trees. Rotten wood, such as an old treehouse or playhouse, also provides a growing surface for the mushrooms.
Dig down beneath the mushrooms to check for any decaying matter underneath, such as old roots or pieces of wood, and remove it.
Mix an organic fungicide compound to treat the mushrooms. Combine 2 tbsp of baking soda and 1 gallon of water in a bucket and apply liberal doses of the mixture to any mushrooms in your yard. This solution is safe to use in your garden. Alternatively, sprinkle baking soda on the mushroom tops and pour water over the baking soda.
Spot-treat the garden when new growth appears. Keep a solution of baking soda and water in a power sprayer or garden sprayer and check for mushrooms every few days.