On your farm, it's possible to find big tree-like mushrooms, but it's not really clear to me when and how they spawn. Do I need to plant normal seeds and they'll randomly turn into mushrooms? Do they have to be "natural" seeds dropped from a grown tree? Do the mushroom seeds spawn on their own on empty tiles?
In short, what is the best course of action in order to get a giant mushroom?
Having looked at the game code, I can now answer this myself. Every day of Fall there's a 5% chance that the Big Mushroom growing event will activate. When it does, the game will randomly select ten "terrain features", which include trees, grass, fences, tilled dirt, fruit trees, and flooring. Any trees that are fully grown and are not tapped, if they're selected in such manner, will become Big Mushrooms.
As such, the best way to get Big Mushrooms is to have a lot of fully grown trees, untapped, and very little of every other item mentioned above, all throughout Fall.
