It's best to have both a map as well as labels on the trees. Labels can get lost, vandalized, or disappear, so map provides a good back up. I like Tomas' idea of embossed metal labels, but i would want to have more than just a number on the latel. It would be great to have the cultivar name, and maybe date planted. If you have only a few plants numbers would be enough, but if you have hundreds you're not going to remember what number 596 corresponds to. Or you would have to have the key with you also at all times, which also defeats the purpose of having labels. Some plants are kind of tricky to attach a label to, like bananas, and pineapples. The best all around would be to have the label on a stake with a sign. Kind of like is used in arboretums. But this gets kind of expensive to do lots that way.