I know of two that seem to be impervious to fading. The nursery probably has aluminum tags. You write on them with a ball-point pen. The indentations are permanent, even if the ink all goes away. That's like the tags on roses. Maybe this can be done with simple pieces of heavy-duty alfoil? Dymo plastic labels are permanent in the same way, because they are embossed and the text remains even if the color vanishes. That requires the embossing machine, however, and is a bit slower if you have many dozens to label.