Acorns, the seeds of oak trees, fall off their parent trees when they mature, which typically occurs between September and early November in the United States. The exact time of maturation varies slightly with different types of oak trees.
Maturation of an acorn takes six to 24 months, depending on species. The color of an acorn changes from green to dark brown as it matures. Acorns may also fall off trees if the branches are shaken.
When the trees are plentiful, acorns are an essential food source for the local wildlife. Birds, rodents and deer commonly eat acorns that have fallen to the ground.