This is a common question because frozen spinach tends to be packed more densely than fresh spinach. The simple answer is yes, green smoothies can be made with frozen spinach and substituted at about a 1:6 ratio. So if your smoothie calls for 2 cups of fresh spinach, use about 1/3 cup frozen instead. Don’t defrost frozen spinach that you use for smoothies; instead toss it right into the blender. You can buy frozen spinach in a bag or freeze your own in ice cube trays for ease of use.