Leucoprinus birnbaumiiIf you have found a mushroom that looks like those below, and was growing a flower pot, chances are it is in fact Leucoprinus birnbaumii.
This is the so called 'flower-pot fungus' as it is a common contaminant of possting mixes. It's harmless to the plant.
Leucoprinus birnbaumii is saprobic; growing alone, gregariously, or clustered in flower pots, greenhouses, and so on--or, in warm conditions, outside in gardens and other cultivated areas; also growing in hardwood and conifer forests, especially in disturbed ground areas such as pathsides. It is common in many countries.
Cap: 2.5-6 cm, oval when young, becoming broadly conical or bell-shaped; dry; powdery to scaly; the margin lined or grooved nearly to the center; bright to pale yellow, often with a darker center.
Gills: Free from the stem; yellow to pale yellow; crowded.
Stem: 3-10 cm long; 1-5 mm thick; more or less equal, or tapering to apex; dry; smooth or powdery; with a fragile, yellow ring that often disappears.
Flesh: Yellow throughout; very thin.
Odor and Taste: Odor not distinctive; taste mild or bitter.
Spore Print: White.
Microscopic Features: Spores 8-13 x 5.5-8 µ; smooth; elliptical; dextrinoid; thick-walled; with a small pore. Basidioles inflated, brachybasidiole-like. Cheilocystidia to about 65 x 15 µ; ventricose or irregular, often with fingerlike projections. Pleurocystidia absent.