Dumpsite

In the scene with Marshall attempting to grow the mushrooms, he cuts a piece from the stalk and puts it in the petri dish. When that fails to produce what he is looking for, he looks at his mushrooms and see 3 and declares there are only 3 attempts left. That isn't how it works. While he could make an attempt the way it is shown, the standard method would be to use the spores, for which there are billions. Even if he needed to use segments of the mushroom, he has many attempts possible.

It is just laziness or ignorance on the part of the writers who present an animated show for adults and demonstrate less research than required when it comes to details. Alternatively, they would need to provide details as to why a single segment is all that is possible for each mushroom to grow the mycelium.