baby lettuce most likely means leaf lettuce, such as Boston lettuce. Not all that good for dragons due to lack of nutrients. Baby greens can be any type of greens, but spinach and romaine are probably popular ones.
I really don't like these pre mixed salads. I find they go bad must faster than buying the greens i need individually. Most often the Boston leaf lettuce or that purple stuff, whatever it's called, will rot first and cover everything in the bag with rotted purple blobs which are a pain to wash off the rest.
I typically just buy 1-4 staple greens, (endive, collard greens, dandelion greens, mustard greens, escarole, chicory, etc) and add other greens, such as romaine, bell peppers, zucchini, other vegetables and occasionally some fruit. Some times I add some kale to the mix, but never as a staple, due to it's high oxate count as well.
I always try to rotate greens monthly, so I am; not always serving the same stuff, this way my dragon doesn't get spoiled or picky on one type of green. Also, not all greens good for dragons is available constantly.
If you are worrying about alot of waste, there are a few ways to keep the greens fresh as possible (washing and laying between paper towel and storing in a container or bag), could always buy more bearded dragons so you have more to feed and thus less waste going to rot. Or buy other green eating animals, such as blue tongue skinks, iguanas, uromastyx, guinea pigs, rabbits, etc.
Or, just eat it yourself. Add a bit of salad dressing and hold back the insects, and a bearded dragon salad is an excellent source of fiber and nutrients for us.
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PHLdyPayne