Wash spinach thoroughly in several changes of cold water to remove all grit and sand. Remove any thick stems. Baby spinach can be frozen whole; mature spinach with large leaves should have stems removed.
Blanch spinach in boiling water for 2 minutes (use a large pot — spinach needs room to move in the water). Transfer immediately to ice water for 2 minutes. Drain and squeeze out as much water as possible. Form into tightly packed balls about the size of a golf ball.
Flash freeze the squeezed spinach balls on a sheet pan, then transfer to freezer bags. Each ball is roughly equivalent to one cup of fresh spinach, which is convenient for recipes.
For smoothie use: you can skip blanching and freeze raw spinach leaves directly in bags. They will be limp when thawed but blend perfectly. Pack leaves tightly into measured portions.
8 to 12 months (blanched). 2 to 3 months (raw, unblanched, for smoothies only).
For cooking: use from frozen. Drop frozen spinach balls into soups, pasta sauces, or scrambled eggs. For recipes calling for thawed spinach: thaw in the refrigerator overnight, then squeeze again to remove excess water.