I'd like to gather more feedback on XP earned in this skill. We have reports that it's much too fast and much too slow. That's probably because the XP is earned per mushroom, and if you properly prepare, it's possible to harvest quite large batches of mushrooms. If you don't prepare properly, the XP earned will be small. There is random fluctuation involved as well -- if you use good fertiliizer and grow in the right moon phase, you can conceivably get hundreds of mushrooms at once (though the chances of that are small).

After you've grown a few dozen crops, please report your general opinion of the XP curve here, with notes on what you're raising and whether you're using appropriate fertiliizer / growing in appropriate moon phases!

Edited to add: the amount of XP earned from some of the substrate recipes is higher than intended, and the amount of XP earned from most of the grow-box recipes is lower than intended. Those will be tweaked in the next update. But in any case, the core way to earn XP here is supposed to be from growing mushrooms in grow-boxes!