Foragers across Europe hold that mushrooms come up after a full moon, and mushroom-picking calendars printed on the same principle still sell.

Fruiting is driven by rain and temperature, and no controlled study has found a lunar signal in it. What the lore may be catching is that a full moon gives enough light to walk a wood before dawn, which is when the best mushrooms are still there. The tradition is about the picker's schedule rather than the fungus.