Osmocote will last indefinitely if kept dry and relatively cool (under 100 degrees F). Each granule is encapsulated so the nutritional value remain intact as long as the coating is not breeched by excessive moisture or undue pressure like from a sudden smack to the package. For example, if your car runs over a bottle in the driveway or garage the coatings of the individual granules may be cracked and thereby compromise the nutrients. So, the short answer is that with reasonable care, any Osmocote you may have on-hand from a partially used package in the previous season should perform quite well. Keep growing, Marianne Binetti.