Leucocoprinus birnbaumii (Corda) Singer (1962) is a common mushroom in flowerpots (Fig 1). It is a saprobe that colonizes rich organic material, including indoor flowerpots. The cap begins as an egg-shaped ball, which grows into a bell-shaped cap, often ornamented with scales. The bright yellow color makes this mushroom easily identifiable, but the cap will often fade to white/beige over time. The spores are ellipsoid with a pore at one end (Fig 2), with a white spore print. L. birnbaumii was reported as Agaricus luteus in 1785, but that species name was already taken by another mushroom, leading to the use of Lepiota lutea for many years, until a taxonomic consensus was reached.
Other notes: L. birnbaumii is not harmful to the plants in the pots it grows in, nor is it poisonous to the touch. Do not eat L. birnbaumii, as it is likely toxic in large amounts.