I have seen in a number of series, when a person is sad, they have mushrooms growing out of their head. In Ouran High School Host Club, this happens a number of times to Tamaki when Haruhi rejects him. He often goes and sits in a corner.
The below gif from FMA shows it as well. Where did this trope come from?
I cannot tell you who started it or where it comes from exactly, but it is based on the fact that mushrooms grow in dark and dank places. Exactly the kind of environment depressed characters are visually and mentally put into.
It is mentioned in the Corner of Woe trope:
Bonus points if you manage to act gloomy enough to attract Ghost Lights or grow mushrooms in your dark, damp corner of woe.
In Japanese, a gloomy/depressed person can be referred to as being "damp" (じめじめ, jimejime). What grows in dark (i.e. gloomy) and damp places? Mushrooms, of course.
The expression from Haruhi there about telling Tamaki to "stop growing mushrooms in one's closet" which means, more or less "stop being depressed".
