Mushrooms in Grass: Are They Bad for Your Lawn?

The fungi that push mushrooms up through turf are decomposers, not parasites of grass. University of California IPM states that these mushrooms are usually harmless to grasses and that the main complaint is that people find them unsightly. Illinois Extension describes the same organisms as feeding on organic matter in the soil.

So the mushroom in the middle of your lawn is not eating your grass. It is eating a buried board, a dead root, or the thatch layer. When the food runs out, the fungus stops fruiting there.

That is the answer for the ordinary case. The rest of this page covers the situations where a lawn fungus genuinely does cause turf damage, and how to tell those apart from harmless fruiting.

A fairy ring is one fungal colony seen from above. It started at a point, spread outward evenly, and now fruits along its leading edge. Iowa State University Extension attributes fairy rings to 50 or more species of soil-inhabiting fungi and notes they favour light-textured soils that are low in fertility and moisture.

Rings show up in three forms, and they tell you different things:

• A dark green band. Harmless, and arguably useful. The fungus is mineralising organic nitrogen and the grass above the colony gets a free feed. Illinois Extension attributes the lush ring to that nitrogen release.
• A ring of mushrooms, with normal grass. Also harmless. You are seeing fruiting without any water problem underneath.
• A brown or thin band. This is the one that needs work. Iowa State attributes the brown grass to drought stress caused by the dense growth of fungal material, and says the weakened grass then becomes vulnerable to environmental stress, disease and weeds.

In a bad ring, the mycelium packs the top few inches of soil so tightly that water sheets off instead of soaking in. Illinois Extension states that dense fungal mats can inhibit water movement into the soil, causing browning, and adds that they may deplete soil nutrients and produce toxic levels of hydrogen cyanide.

You can test this in under a minute. Pour a cup of water on the brown band and a cup on healthy turf two feet away. If the water beads and runs off the band while it disappears into the good turf, the soil is water-repellent and the grass is simply thirsty.

The fix follows from the diagnosis:

• Aerate through the band. Illinois Extension recommends aeration to break through dense fungal mats so water and air can penetrate.
• Force water in. Illinois Extension describes using a root-feeder attachment on a garden hose to pump large quantities of water down into the ring.
• Feed the ring. Extra nitrogen on the affected area helps mask the contrast and supports recovery, and UC IPM lists nitrogen fertiliser as a way to hasten decomposition of the buried organic matter.
• Last resort. Iowa State notes extreme cases may require digging out the affected sod and replacing the soil.

If tan to honey-coloured mushrooms come up in a tight cluster at the base of a live tree, over a buried root, or around an old stump, that is a different problem. UC IPM describes Armillaria root rot as producing clusters of several to dozens of light brown mushrooms at the base of infected trees and shrubs, sometimes arising from a buried root out in turfgrass away from the tree.

The supporting signs are underground: white sheets of fungal tissue (mycelial fans) beneath the bark, and black shoestring-like strands called rhizomorphs. UC IPM also cautions that mushrooms may never appear on some infected trees, so their absence proves nothing.

Armillaria attacks woody roots, not turf. The grass will be fine. The tree may not be. Have an arborist look at it rather than treating the lawn.

Do not reach for a fungicide first. Illinois Extension states there are no recommended fungicides for controlling these species. UC IPM says it is generally faster and more effective to manage fairy rings in a home lawn with cultural practices than with a fungicide. Iowa State adds that where labelled products exist, suppression may only be temporary.

Do not treat a green ring as a disease. Darker, faster-growing grass is a fertility effect. Feeding the rest of the lawn evenly hides the contrast better than any spray.

Do not assume harmless to grass means harmless to people. The two questions are unrelated. A mushroom can be completely irrelevant to your turf and still be dangerous if a toddler or a dog eats it. See poisonous lawn mushroom types before you decide the patch can wait.

The fungus was already there; the weather made it fruit. Scotts lists three conditions that bring mushrooms up: moisture, shade or cloudy weather, and rich organic material in the soil. Illinois Extension notes lawn mushrooms and puffballs appear especially after heavy rains while they feed on organic matter underground.

So a week of rain after a dry spell, a newly shaded corner under a growing tree, or an old stump rotting under the turf can all produce a sudden crop. When the rain stops and the soil dries, most of the mushrooms disappear on their own.

Sometimes, but not always. Rain alone is enough to trigger fruiting. The sign to look for is how long the ground stays wet: Scotts points to lawns where water stands or the soil stays damp long after rain, which usually means compacted soil and poor drainage.

• Mushrooms only after rain, gone in a few days: normal, no change needed.
• Mushrooms after every irrigation cycle: water less often and more deeply, early in the day.
• Soggy patches that never dry: aerate to relieve compaction.
• Thick spongy thatch: Scotts recommends removing thatch deeper than half an inch, because it holds moisture.

Mostly good for the soil. Scotts describes lawn mushrooms as a sign of organic-rich soil and says they break that material down and make the soil more productive. Holden Forests & Gardens adds that many yard fungi, such as boletes and Russula species, partner with tree roots and help trees take up nutrients, and its advice is to leave lawn mushrooms alone in nine out of ten cases.

The exceptions are specific:

• Honey-coloured clusters at the base of a living tree can mean Armillaria root rot (see the section above).
• Children or pets use the lawn and might eat them. Some lawn species are poisonous; see poisonous lawn mushroom types and dog ate mushroom in yard.
• A bright yellow foamy blob is usually dog vomit slime mold, not a mushroom; Holden Forests & Gardens says slime molds usually go away within a few days.

There are two timescales. For today, Illinois Extension advises breaking or mowing off mushrooms when they first appear if children or pets might contact them. For the long run, remove what the fungus is eating and the conditions it needs.

• Pick, mow or rake them up and put them in the trash.
• Dig out buried wood, old roots and stumps where many mushrooms appear (Illinois Extension; Scotts).
• Aerate and dethatch so the soil drains (Scotts).
• Let in more sun by thinning branches over shady patches (Scotts).
• Clean up pet waste, which Scotts notes can also bring mushrooms up.
• Skip the fungicide. Illinois Extension lists none for these species.

Step-by-step detail is on how to get rid of mushrooms in a lawn.

• Scattered small mushrooms after rain, grass looks normal. Leave them or mow them; they will fade as the lawn dries.
• A ring of mushrooms or a dark green circle. A fairy ring. No action needed unless the grass inside browns.
• A brown, dry band in a ring. Water-repellent fungal mat. Aerate and water deeply through the band.
• Clusters at a tree base or on roots. Possible Armillaria. Call an arborist about the tree.
• Large white mushrooms where children play. Remove them with gloves and bin them; see white mushrooms in yard.
• A child or pet ate one. Call Poison Control at 1-800-222-1222 (U.S.) now, before symptoms start, or call your vet or ASPCA Animal Poison Control at (888) 426-4435.