Mushrooms are the reproductive structures of certain fungi. The gills under the cap of a mushroom provide a surface from which millions of spores are released into the environment. The spores drift on air currents, and with a little luck, a spore will land in a place favorable for germination. Once a spore germinates, it can grow to form a new fungal colony. Mushroom-forming fungi grow by means of threadlike filamentous cells called hyphae.
Fungi are important decomposers in the environment. Fungal hyphae exist unseen under the turf most of the time, where they seek out dead organic matter. They break down organic substances (dead leaves and roots, wood, animal waste, etc.) to recycle and and return nutrients to the soil.
Mushrooms in the Yard
Although some people consider mushrooms to be an unsightly addition to a well-groomed lawn, they are not harmful to the grass. The mushrooms don't last long and the majority of the fungus remains invisible underground most of the time, performing its useful role as a decomposer.
Some mushrooms that appear in yards may even be edible, but should not be eaten even if they are positively identified. This is because pesticide residues may be present in the mushrooms at an unsafe concentration.
Although yard mushrooms are harmless to the grass, they may be poisonous to people or their pets. This is another reason why yard mushrooms should not be eaten. Dogs sometimes eat wild mushrooms, so if a pet is present, it is best to pick and dispose of any mushrooms just to be on the safe side.
Conditions for Mushroom Growth
When conditions are right, the underground fungus initiates a reproductive cycle and produces mushrooms. Several conditions must be met before this can happen.
Mushrooms are the product of sexual reproduction, so with few exceptions, two compatible individual fungal colonies must meet and fuse together to eventually undergo meiosis.
Environmental conditions must also be favorable. Mushrooms grow quickly and require sufficient water and humidity to complete development. This is why mushrooms pop up in a yard within a day or two of a good rain. Areas covered with newly laid sod or freshly sown grass seed often produce mushroom because of the frequent watering required to establish the turf. If the water runs out or the air is too dry, the mushrooms will abort.
A cold shock also enhances fungal reproduction. A sudden cold front, in combination with rain and lingering humidity, is likely to trigger a flush of mushrooms to appear on a lawn.