After an autumn rain, they suddenly appear: mushrooms. They pop up in the lawn, in the mulch around the base of trees, and among the perennials.
Some homeowners are alarmed by them, but mushrooms should be a welcome sight. They’re delivering good news about the health of your soil.
“Mushrooms mean fungi,” said Meghan Midgley, a soil ecologist at The Morton Arboretum in Lisle. And fungi are one of the major ingredients of healthy soil that is good for your plants.
“Fungi are natural composters,” she said. They do most of the work of breaking down each year’s batch of fallen leaves. “If you see mushrooms, it’s a sign that your soil has a healthy soil food web,” Midgley said.
Along with other microorganisms, fungi consume all kinds of organic matter — the remains of dead plants, animals and other living things — and release useful nutrients into the soil to be absorbed by plant roots.
Gardeners might wonder why they would want a fungus in their yards, when fungi are the source of plant diseases such as powdery mildew and cedar-apple rust. But though some kinds do cause disease, far more fungi are beneficial. They are essential to good soil, thriving plants and healthy ecosystems all over the world.
A mushroom is actually only a small, fleeting phase of a soil fungus. Usually, the fungus is hidden underground, a network of microscopic filaments that twine between soil particles and plant roots. That network can grow for many years, extending for hundreds or thousands of feet under the soil.
When conditions are just right — often in the fall, after rain — underground fungi may send up those conspicuous mushrooms. Why? To make more fungi. The mushroom is a fruiting body, full of minuscule reproductive spores that are light enough to float on air.
A typical mushroom’s stalk lifts the spores in its cap above the soil just high enough to be caught by a tiny breeze. Wafted away, a spore might land on fresh soil, where it can start a new fungus network.
Mushrooms are temporary. They last only long enough to release their spores and then quietly dry up or dissolve over a few days.
They do no harm to any of the living plants in your garden, said Sharon Yiesla, plant knowledge specialist in the Arboretum’s Plant Clinic. “If they annoy you, you can rake them away,” she said. “Just don’t try to kill them with any kind of chemical. That would damage the fungi in your soil, and those fungi are good for your plants.”
In some yards, mushrooms appear again and again in the same spot, often in a ring, and then fade away. Once, people thought these evanescent rings were magic and called them “fairy rings,” imagining them as enchanted places where forest spirits gathered to dance.
The scientific truth is simpler. “The ring means there’s something big and woody underground that is being broken down by a fungus,” Midgley said. “For example, it could be the remains of an old tree stump.”
Consuming a large piece of wood can be the work of years for a mass of fungal fibers, called a mycelium. Meanwhile, the fungus will try to reproduce from time to time by sending up mushrooms. Since mushrooms tend to be produced at the perimeter of the mycelium, they appear above ground in a ring.
Mushrooms have a reputation for being dangerous, and it’s true that a few species are toxic to eat. The vast majority of mushrooms are harmless, but they are difficult to identify for certain, so it’s not a good idea to eat any you find in your yard. “Play it safe and get your mushrooms at the grocery store,” Yiesla said.
The only time mushrooms might be a sign of trouble is if they are growing at the base of a tree or on the bark. That could mean that there is decay in the wood of the tree. “Have the tree checked out by a professional arborist,” she said.
If mushrooms appear anywhere else, welcome them as an indicator of healthy soil.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (mortonarb.org/plantadvice or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
