Grass can spread to dirt tiles next to grass tiles as fast as within 24 hours real life time. Grass can also spread from a tree tile to adjacent tiles.
Planting flowers or [[thatch]] on a dirt tile yields flowers and grass on success. This can be useful for keeping pens grassy to feed animals like [[cow]]s, [[bull]]s, [[horse]]s and [[deer]]. This also builds your [[gardening]] skill without wasting valuable [[sprout]]s. Note that animals can quickly graze or stomp the tile into [[packed dirt]] though, losing the flowers and grass.
Planting a sprout also creates a tile that looks grassy, but animals cannot graze on tree tiles. If the tree or bush is cut down, the tile turns into a grass tile.