Sunflower

Sunflowers are large plants which can be found growing naturally on grass, and can be freed with a pickaxe or drill, or by coming into contact with water. They cannot be grown from seeds like most other plants, but are instead placed fully-grown, either naturally upon world generation or by the player. They can also be purchased from the Dryad for each, while a Blood Moon is not occurring.

Sunflowers provide the following functions:
 * The two tiles directly underneath them cannot become Corrupted, Crimsoned, or Hallowed, even in Hardmode. In some cases, this can prevent Corrupted and Crimson grass from spreading sideways.
 * Each Sunflower decreases an area's total Corruption rating by 5 tiles. This rating is used to determine whether NPCs will reject housing. If placed in large quantities, they can allow NPCs to live near tainted blocks. This effect covers a radius of about 25 tiles.
 * They provide the Happy! buff to players within a centered on the Sunflower, which grants a +10% movement speed bonus and 17% reduction in enemy spawn rate.
 * Sunflowers help to prevent Graveyard biomes from forming by canceling out one Tombstone each.

Sunflowers can only be placed on regular, Hallowed, or Jungle grass, at any height. They cannot be placed on Corrupt grass, Crimson grass, bare Dirt Blocks, nor in Clay Pots or Planter Boxes.

Trivia

 * In the release trailer for 1.3, a variant of sunflower was shown with a golden interior as opposed to the regular sunflower (some are seen in the beginning of the trailer). It never ended up in the final update and its effects are unknown.
 * Sunflowers' ability to resist Corruption and Crimson may be a reference to their real-world reputation for cleaning up nuclear radiation.
 * A sunflower is used as the loading icon on the versions.
 * The sunflower in the loading icon is very similar to the loading icon of Plants vs. Zombies 2, a marigold that plays a very similar animation.
 * The Happy! buff sprite seems to have a frown rather than a smile.