Flower on Head

In manga and anime, sometimes a flower sprouts out of a character's head as a manga symbol, meaning they're merry, carefree, happy-go-lucky, having no worry in the world, peaceful. It's used when a character is in love, or when they're being silly, stupid, as it means they've stopped being serious.

Patalliro, パタリロ, example of character with a flower sprouting out of his head.

Character: Patalliro, パタリロ
Anime: Patalliro!, パタリロ！ (Episode 2)

Meaning

This symbol is related to a few Japanese expressions:

atama ni ohana ga saite-iru
頭にお花が咲いている
A flower is blooming on [someone's] head.
- Meaning:^{(detail.chiebukuro.yahoo.co.jp:頭に花が咲いている)}
- ukarete-iru
  浮かれている
  To be floating. (literally.)
  To be in good mood. To be merry. To be in high spirits.
- ukareru
  浮かれる
  To float.
  To become in good mood. (future tense.)
  (intransitive, unaccusative, eventive verb.)
- uku
  浮く
  To float.
  (transitive, lexical causative verb.)
- atama ni hana ga haete-iru
  頭に花が生えている
  A flower is sprouting on [one's] head. (literally.)
  (same meaning.)
atama no naka ga ohana-batake
頭の中がお花畑
Inside [one's] head is a flower field.
- Meaning that someone's mentality is extremely peaceful, like one of those peaceful flower fields, with a gentle breeze making the flowers calmly swing.
- Used when someone is in very high humor, like when they've fallen in love, thinking about things romantically in general, or, like the above, when they have a gentle personality.
- Most likely, a flower literally sprouting on the character's head symbolizing that inside of the head is a flower field.
{mawari ni ohana ga tonde-iru} hito
周りにお花が飛んでいる人
Someone [who] {flowers float around}.
- tobu
  飛ぶ
  To jump. To fly. To float. (to not be in contact with the floor.)
- Meaning that someone is calm, gentle, fluffy, has a warm, comforting personality.^{(chiebukuro.yahoo.co.jp:周りに花が飛んでる人)}
- In anime, such characters tend to talk slow and, well, think slow sometimes. They're also likely to be drawn with sweat drops flying away from them when they feel nervous.

Examples

For reference, som examples of flowers sprouting on characters' heads:

Character: Patalliro, パタリロ
Anime: Patalliro!, パタリロ！ (Episode 2)

Context: an idiot being more idiotic than normal.

Shinomiya Kaguya 四宮かぐや with a flower sprouting out of her head.

Character: Shinomiya Kaguya 四宮かぐや
Anime: Kaguya-sama wa Kokurasetai? ~Tensai-Tachi no Ren'ai Zunousen~ かぐや様は告らせたい？～天才たちの恋愛頭脳戦～ (Season 2) (Episode 2)

Context: the baka version of Kaguya, called Bakaguya in English.
sudare-me
すだれ目
"Window-blind eyes." Eyes drawn using only parallel lines.

Exceptionally:

Left: Shindou Yuuta 進藤ユータ
Right: Kamakura Shiho 鎌倉志保
Anime: WWW.WORKING!! (Episode 12)

Context: Shiho sprouts a flower on her head, symbolizing that she's entered a happy-go-lucky mode and talking silly. Yuuta plucks the flower and tells her to stop the silliness. She then asks him a serious question instead, prompting him to try to put the flower back to avoid the question question. Before he manages to put it back, she tells him:
ohana φ modosanaide Yuuta-kun.
ohana-batake wa toriaezu shuuryou yo
お花戻さないでユータくん
お花畑はとりあえず終了よ
Don't put the flower back, Yuuta-kun.
The flower field [part] has ended for now.
- modosu
  戻す
  To return. To put something back where you took it from.

Non-Symbol

Sometimes, a flower springs out of a character's head and it isn't used as a symbol that disappears momentarily like an anger mark, but as a physical flower actually growing on their head.

Hedoro 屁怒絽, example of character with a flower sprouting out of his head.

Character: Hedoro 屁怒絽
Anime: Gintama 銀魂 (Episode 47)

Context: Hedoro, one of the most powerful warriors in the universe, was infected by a parasitic flower that sprouts out of his head, turning into a peaceful florist.

Ryoutsu Kankichi 両津勘吉, with a flower growing on his head.

Character: Ryoutsu Kankichi 両津勘吉
Anime: Kochira Katsushikaku Kameari Kouenmae Hashutsujo こちら葛飾区亀有公園前派出所 (Episode 340)

Context: this whole episode is about a flower having grown on the protagonist's head.
Hana no Ko: Ryou-chan
花の香・両ちゃん
Flower child: Ryou-chan.
(episode title.)

Often, instead of a flower sprouting on a character's head, flowers are drawn as background effect to symbolize their peaceful or lovable personality.

Harpy, ハーピィ, wearing overly long sleeves (moe-sode 萌え袖) that are also detached sleeves, デタッチドスリーブ, with flowers in the background.

Character: Harpy, ハーピィ
Anime: Maoujou de Oyasumi 魔王城でおやすみ (Episode 5)

Context: Harpy is a monster girl that loves girls' talk, including talking about love and romance. She has wings, feather ears (hanemimi 羽耳) and uncute claws that she hides with detached overly long sleeves (moe-sode 萌え袖).

When mushrooms sprout on a character's head, that instead means they're feeling down, depressed.

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)