By late summer, plump green hulls hang heavily in the walnut canopy. As the hulls slowly split, they reveal promising peeks of brown shells. By early fall, the hulls release their cargo and brown shells plop to the ground. On windy days, some nuts may drop still encased in their hulls.
In our area, the general order that acorns ripen in the fall is valley oak, black oak, tanoak, coast live oak, and then canyon oak. The acorns of species in the white oak group take 6 months to ripen (pollinated in spring, full acorn formed in subsequent fall), whereas acorns of the black oak group take 18 months to grow and ripen. On branches of the black oaks, you will find first-year acorns looking like shingled buttons, and second-year acorns fully formed. The first crop of acorns dropped is usually the damaged acorns, ones with weevils or mold or otherwise underdeveloped. They are still good eating for wildlife, but if you are collecting acorns to grow oaks, wait a few weeks for the second drop when the healthy, heavy acorns are finally released from their caps and fall and roll to their future destination.