Mushrooms in your lawn or garden are usually harmless and often beneficial. Warren Rohner photo.

At this time of year, clients often ask us about mushrooms that have sprouted in their lawn or garden beds. Since this is a common problem, we are updating and reposting a blog post from last year.

Should you be concerned?

In most cases, the mushrooms are completely harmless, and they provide concrete benefits to your yard and plants. If you have children or pets, it’s best to err on the side of caution and remove the mushrooms.

Beneficial mushrooms 

According to Michael Kuo at MushroomExpert.com, there are two types of beneficial mushrooms:

• Saprobic mushrooms help decompose dead organic material. Their mycelia chew up fallen leaves, dead blades of grass, woody debris, dead rootlets and so on. Imagine what would happen if nothing decomposed in your yard or garden: stuff would eventually pile up sky high! Yes, you can remove dead organic matter yourself. But your rake merely carries leaves away. Mushrooms break them up into natural particles that enhance the soil.
• Mycorrhizal mushrooms are involved in symbiotic relationships with plants and trees. The mushroom’s mycelium surround and protect plant’s rootlets, and they help the plant absorb water and nutrients. In exchange, the fungus gets sugars and amino acids. Most trees cannot survive without mycorrhizal partners from the fungal world.

Fairy ring fungi

Fairy ring. Clemson University – USDA Cooperative Extension Slide Series, Bugwood.org.

Fairy ring is common in lawns. The mushrooms are visible in spring and summer. Several different types of fungi may cause fairy rings. Typically, fairy ring is not detrimental to lawn health.

The only control method we recommend is to remove the mushrooms when they appear to prevent the fungus from spreading. You can collect and compost them, knock them down with a rake or hoe, or mow them over with your lawnmower.

You can mask the symptoms of fairy ring by using a nitrogen fertilizer to green up the lawn. We also recommend watering deeply and infrequently to help build lawn health.

Harmful mushrooms

Armillaria root rots. Linda Haugen, USDA Forest Service, Bugwood.org.

Not all mushrooms are benign. Some grow from harmful fungi.

Armillaria is a root rot that has honey colored mushrooms. This fungus is naturally occurring in the Northwest.

Armillaria root disease is the most common and most widely distributed forest root disease in Oregon and Washington. In western Washington, it is primarily found in trees that have been stressed.

An infected tree will commonly show a gradual decline, with small, yellowish leaves, reduced growth and dieback of branches. Trees that are chronically infected may wilt suddenly during periods of stress, such as drought.

It is easy to identify armillaria by looking for:

• Honey-colored mushrooms found at the base of the plant (see photo)
• Black shoestring-like structures (called rhizomorphs) found under the bark, around roots or in the soil
• White fungal mats under the bark

Mushrooms of armillaria may develop annually in the fall during wet periods. They usually appear in groups, with up to several hundred on or near decaying wood, stump, or roots. Spores from these mushrooms can infect recently wounded plants. However, they apparently do not play an important role in the spread of the disease.

Note: many other fungi also produce mushrooms on or near decaying wood.

The best way to prevent armillaria root rot on ornamental trees and shrubs is to keep the plants healthy. Water thoroughly during drought periods and provide good drainage. Avoid injuring trees and avoid damaging or stressing the roots.

To find out more

Here is an article from Missouri Botanical Garden with more detail about fairy ring.

To learn more about armillaria, check out these and other sources:

• US Forest Service
• University of Minnesota Extension 
• University of California IPM Program
• University of Illinois Crop Extension

If you have a serious interest in mushrooms and fungus, a good field guide for the amateur mycologist is Mushrooms Demystified by David Arora. For local mushrooms, a good field guide is Mushrooms of the Pacific Northwest by Joe Ammirati and Steve Trudell. Or find a list and photos of edible mushrooms on the Northern Bushcraft website.