Mushrooms Growing on Head

In manga and anime, sometimes mushrooms start growing on a depressed character's head, or the character starts growing mushrooms in some dark, lone corner.

atama ni kinoko ga haeru
頭にキノコが生える
Mushrooms sprout on [someone's] head.

Alphonse Elric アルフォンス・エルリック, Winry Rockbell ウィンリィ・ロックベル, and Edward Elric エドワード・エルリック, the latter with mushrooms growing on his head.

Left: Alphonse Elric アルフォンス・エルリック
Middle: Winry Rockbell ウィンリィ・ロックベル
Right: Edward Elric エドワード・エルリック
Anime: Fullmetal Alchemist: Brotherhood, Hagane no Renkinjutsushi 鋼の錬金術師 (Episode 21)

Explanation

In Japanese, the mimetic word jimejime ジメジメ means somewhere "damp," "humid," but it can also mean someone "melancholic," "gloomy."

Since mushrooms grow in places that are dark and damp (jimejime), it only makes sense that they also grow on people that are gloomy and melancholic (jimejime).^{(detail.chiebukuro.yahoo.co.jp)}

Konno Junko 紺野純子, growing mushrooms on her head.

Character: Konno Junko 紺野純子
Anime: Zombieland Saga, ゾンビランドサガ (Episode 7)

Completely unrelated to that: sometimes mushrooms sprout from characters because those mushrooms are parasites. For example, the pokémon Parasect is a crab that's controlled by a parasitic mushroom.

Also completely unrelated: in Japan, mushrooms are a sexual innuendo.

Examples

For reference, some examples of characters with mushrooms growing out of their heads or bodies.

Mostly sulking alone in a dark, sad corner.

Suou Tamaki 須王環 growing mushrooms in a lone corner, dubbed "mushroom room," kinoko no heya キノコのへや, and Fujioka Ryouji 藤岡涼二.

Left: Suou Tamaki 須王環
Right: Fujioka Ryouji 藤岡涼二
Anime: Ouran High School Host Club, Ouran Koukou Hosuto-Bu 桜蘭高校ホスト部 (Episode 10)

Context: Tamaki sulks in taiiku-zuwari 体育座り position, i.e. sitting on the floor holding his knees.
kinoko no heya
キノコのへや
Mushroom room.

Nakano Azusa 中野梓, example of bunny ears, usamimi うさみみ.

Left: Akiyama Mio 秋山澪
Right: many irrelevant characters
Anime: K-On!, Keion! けいおん！ (Episode 9)

Anime: Soul Eater (Episode 25)

Context: Soul Eater ソウル＝イーター warns Maka Albarn マカ＝アルバーン about the dangers of not going outside for too long.
itsumademo uchi no naka de kusubuttetara...
いつまでも家の中でくすぶってたら･･･
If [you] keep secluding [yourself] inside [your] home...
atama kara kinoko ga haechimau zo
頭からキノコが生えちまうぞ
Mushrooms will sprout from [your] head.

When a flower sprouts on a character's head instead, that means they're being peaceful, in love, or silly.