Cup mushrooms (Peziza domiciliana), also known as domicile cup fungus, are a common fungi that afflict lawns. While not harmful to plant life, they are unpleasant to look at, with their upturned, cup-shaped tops. They can also be potentially dangerous as children and pets may accidentally ingest them. To rid your lawn of these cup fungi use cultural management to make growing conditions inhospitable for the fungi. To kill established fungi, you can use a high nitrogen fertilizer to kill organic matter that feeds the fungi.

Wild mushrooms growing on a forest log
credit: RUJITOP/iStock/Getty Images
Cup mushrooms thrive on dying organic matter and shady, moist conditions.

Fertilize Your Mushrooms to Death

For established fungi living on buried or above-ground organic matter, such as a buried log or a tree stump, you can apply nitrogen fertilizer to kill mature mushrooms. According to the University of California Davis, the amount of fertilizer needed depends on the quantity of nitrogen. You need 1/10 of a pound of nitrogen per 100 square feet of lawn. This would mean 1/2 pound of water-soluble fertilizer that is 21-0-0. Apply the fertilizer directly to the mushrooms, and do not use a slow-release fertilizer. Do not apply more than once every two weeks, as you could otherwise harm your lawn. Fertilizer can be applied any time during the growing season, but choose a period with little precipitation to avoid washing away fertilizer.

Reduce Organic Matter

The amount of decomposing organic matter can affect the growth of cap mushrooms. Tree stumps and buried logs are common sources of fuel for fungi, and can increase your chances of developing cup fungi. Apply a high-nitrogen fertilizer directly to difficult-to-remove organic matter, such as stumps, to hasten the decomposition process. For organic matter that has already started decomposing, applying fertilizer once should be sufficient. Use 1 cup of undiluted fertilizer for every 6 square inches of organic material. For large branches or logs, remove the logs from your lawn as soon as they become apparent so that the fungi do not have time to take root.

Reduce Soil Moisture

Over-irrigation can lead to the damp growing conditions that are ideal for cup fungi. Practice infrequent, deep watering to encourage grass to develop a deep root system, which reduces water buildup in your lawn or garden. For gardens, apply a 3- to 6-inch thick layer of mulch each year to minimize your chances of having a sodden garden bed. Add compost to your soil before planting in your beds to improve overall soil structure, so there is less likelihood of compaction over time. To reduce soil compaction in your lawn, aerate it regularly. Do so during early fall for cool-weather grasses and in the late spring for warm-weather grasses. Thatch — dead grass that collects at the soil surface — can also lead to water retention, which can increase soil moisture. Dethatch your lawn during the same time that you would aerate your lawn.

Prune Shade Plants

Cup mushrooms enjoy low-light conditions, which help preserve the cool, shady growing environments they prefer. To minimize shady conditions, regularly trim your trees. As a general rule of thumb, only cut off smaller branches — less than 2 inches in diameter — and do not over trim. Removing more than a quarter of a tree's total leaf-bearing branches can be detrimental to the tree's health. Trimming shade trees will not reduce or kill mushroom fungi on its own, so pair this maintenance technique with other lawn care methods.