Have you found a small yellow mushroom growing with a houseplant? It is quite likely to be a mushroom native to tropical areas but found throughout the US with potted plants.
This mushroom known as Leucocoprinus birnbaumii
Leucocoprinus birnbaumii
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Where did it come from?
These mushrooms are native to the tropics however they are now dispersed globally. The fungus is likely established in the soil of tripical houseplants. The fungus can live in the soil without creating mushrooms so it may go undetected. Heavy watering seems to promote mushroom fruiting.
How to kill mushrooms in houseplants?
There is much more to a mushroom than what you see above ground. Mushrooms grow from an interconnected web of mycelium that is often invisible to the naked eye. Unlike with plants, pulling out a mushroom has very little impact on the mycelial ‘roots’ which can regenerate from the smallest amount of tissue. Killing fungi in soil is incredibly difficult. One might try applying a fungicide, repotting in new soil, and reducing watering frequency.
The mushroom is not harmful to the plant, and is only harmful to humans if eaten. If one is really concerned about pets or children eating the mushrooms, they could remove the mushrooms when they appear but expect them to return again in the future.
Edibility of Leucocoprinus birnbaumii
Toxic however so small and insubstantial that it is very unlikely one would eat enough to cause serious issues