I have found that using vinyl-like flexible plastic labels (100-150mm X
12mm) is the best. Then I use a very soft lead pencil (5H) to write on the
label. In the African sun, they last well over 5 years.

I use the added precaution of burying one marker deep in the bag and the
other one is allowed to show. That way if I lose the "showing" marker I
simply unearth the bulb and locate my hidden marker below.

Regards
Greg