Does my soil have mold?

White threads or fuzz in mulch, compost or garden soil

Fine white threads running through wood chips, compost or soil rich in organic matter are fungal mycelium: the feeding network of fungi that are decomposing that material. They show up after rain or watering and fade when things dry out. They are not a plant disease. Leave them alone, or rake the mulch to let it dry if the look bothers you.

White mold on potting soil indoors

A white fuzzy layer on the surface of a houseplant pot is usually a harmless fungus living on the organic matter in the mix. It is a sign the soil stays wet and the air is still. Let the top inch dry out between waterings, give the plant more light and airflow, and scrape off the top layer if you like. It returns if the watering does not change.

Slime molds: the "dog vomit" on mulch

Bright yellow, orange or gray blobs that appear overnight on mulch or a lawn are slime molds. University of Arizona Cooperative Extension notes that slime molds are present in almost all soils, are not parasitic and do not cause plant disease. They are most active in cool, wet weather and disappear when it turns hot and dry. Scoop them up or let them dry out; they do no harm.

Mushrooms in the garden

Mushrooms are the fruit of fungi living in the soil, often feeding on old roots, buried wood or mulch. They mean the soil is alive and moist. Many are poisonous to people and pets, as the Arizona extension guide points out, so pick them off where children or dogs play, and never eat a wild mushroom you have not had identified by an expert.

When mold is a problem: white mold disease

One mold to take seriously is white mold, caused by the fungus Sclerotinia. It rots the stems of many flowers and vegetables, including beans, carrots, squash and tomatoes, and shows as fluffy white growth on stems in humid weather, with tan, brittle stem sections. Arizona Extension notes its hard black resting bodies can survive in soil for up to five years, so it tends to come back. Remove infected plants (not into the compost), clean tools, water the soil rather than the leaves, space plants for airflow, and choose resistant varieties.

Damping-off, where seedlings collapse at the soil line, is another fungal problem linked to soil that stays wet. Soggy ground makes most of these worse; see where soil is most poorly drained.

Is moldy soil a health risk?

For most people, ordinary garden soil and compost are not a concern. The fungus Aspergillus, common in soil and decaying plant material, can cause serious lung infections in people at high risk. The CDC lists people with weakened immune systems (after an organ or stem cell transplant, or during chemotherapy), people with chronic lung conditions, and people with asthma or cystic fibrosis as most at risk, and advises them to avoid activities with close contact with soil or dust.

If that describes you or someone you garden with, ask your doctor about gardening at all, wear gloves, long sleeves and pants, and avoid turning compost or moving dusty mulch; the CDC suggests an N95 respirator where dust cannot be avoided, while noting these steps are recommended rather than proven.