What's With These Things Growing On My House Plant Soil

Sometimes our house plants (or the plants that we bring inside before frost) may have what our clients refer to as "odd looking things sprouting from the soil."  In the Plant and Insect Diagnostic Clinic, we often receive these specimens for identification (See picture below of the most recent one we received). The picture does not do the fungus justice, as it is normally quite beautiful.  (See pictures on the mushroom expert website: Geastrum saccatum; Geastrum triplex.)
This specimen is a mushroom closely related to the puffball mushrooms. This particular mushroom is commonly known an Earthstar mushroom (common name) or Geastrum spp (scientific name).
Often our client's main concern is how they can get rid of this organism from their houseplant pot. Our common answer: eradication is difficult, if not impossible. The reason that eradication is difficult is that the mushroom sprouting from the soil is only the fruiting body. What we don't see is a microscopic net of mycelia (thread-like structures) in the soil, which is associated with the mushroom almost like a set of microscopic roots. The net of mycelia may be associated with the roots of the plant growing in the pot. In most cases the mushroom, and its mycelia are helping the plant in a symbiotic relationship. The word symbiont comes from the word symbiosis, which in the Greek means "living together".
There is a long list of beneficial mushrooms that live associated with plants. The thread-like structures will grow around the roots or inside the root tissue. Ultimately, the fungal hyphal network is formed on the plant roots and aids water and nutrient uptake, helping the host plant to survive adverse conditions. In exchange, the mushroom (fungal symbiont) is provided with carbohydrates or other nutrients. For more information on mycorrhizae visit Extension.Org or OhioLine.

Earthstar mushroom
Fungal bodies of Geastrum spp, known as earthstar mushroom.

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