Beginning in late August and continuing through December - right now - various species of oak acorns are maturing and ripening for collection. Ripening dates vary from year to year and from state to state by as much as three to four weeks, making it difficult to use actual dates to determine maturity.
The best time to collect acorns, either off the tree or from the ground, is when they begin falling - just that simple. This can start as early as late September or as late as the first week in November, depending on oak tree species and location within the United States. The acorn is perfectly ripe when you can easily remove the cap.
Read this report on how to select, prepare and plant acorns for growing oaks.
Acorn Sprouting an Oak Tree - Getty Images/Don Ferrall