Rainy weather can cause mushrooms to sprout in lawns and mulch. “That’s fine,” said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. “What’s not fine is when you see mushrooms sprouting on trees.”
When a mushroom is growing on the trunk or roots of a living tree, or in the soil right around a tree, it indicates that there is a fungus growing in the tree’s wood. Decay-causing fungi can weaken the tree’s wood and make it structurally unsound.
“The mushroom is an alert, telling you to have an expert arborist come out and inspect the tree,” she said. “A trained, certified arborist can judge whether there is any cause for concern.”
Mushrooms are the short-lived fruiting bodies of fungi. Their function is to distribute microscopic spores that are carried away by wind or water to start fungi growing in a new site. They only appear when conditions are right for the fungus to reproduce, usually when it’s moist.
Though the mushrooms themselves are temporary, the fungus they grow from is there all the time. It’s a network of microscopically slender threads called mycelia growing through soil, in mulch or leaf litter on the soil’s surface or in the wood of a tree. If you see fine white threads growing in mulch, they are mycelia.
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Fungi are not necessarily a bad thing. They are an essential part of every ecosystem and an important ingredient in good soil. 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.
If you see mushrooms growing in your lawn, mulch or garden beds, it’s an indication that your soil is healthy and rich with living things. “It means your soil has a good food web,” said Spencer Campbell, the Arboretum’s Plant Clinic manager.
Fungi also play an important role in breaking down dead wood and leaves in forest ecosystems. You’ll often see mushrooms growing on fallen trees in the woods, along with shelf fungi, which stick out like half of a plate. “They are a natural part of the cycle of life and death,” Campbell said. “They slowly consume dead wood and fallen leaves, enrich the soil, and make space for new plants.”’
But a yard next to a home and a street is not the woods. In that situation, where a weakened tree might fall or drop branches on a house or car, mushrooms on a tree’s trunk or roots may be a sign of a hazard. “It doesn’t necessarily mean that the tree needs to be cut down, but it does mean you need to enlist an expert to find out what the situation is,” Yiesla said.
Mushrooms are nothing to worry about when they sprout here and there in the lawn or garden beds, or grow in a clump or ring well away from a tree. They do no harm to any of the annuals, perennials, shrubs or healthy trees in your garden.
“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.” When mushrooms are growing right at the base of a tree or on its bark, that’s another matter.
Mushrooms that march through the lawn in a line that leads to a tree may be growing along an underground root. “Get that checked out,” Yiesla said. “Make sure your tree is safe.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.