Ronco food dehydrator, or, oven set at the lowest setting. I dry and powder greens, weeds, and cactus and sprinkle it over my Sulcatas' hay, dampening it slightly to make everything stick.
You can also dry the greens very slightly in a warm oven and layer in between paper towels and store in the fridge. This allows you to still feed as a primary food item rather than a powder sprinkled over other foods. I don't think that totally dried spring mix would work as a primary food source. By drying slightly they stay good longer, but dried greens never rehydrate back to the point before they were dried.
I also dry flowers such as rose buds to rehydrate later for a treat or snack.