"......... I have taken the bird by the horns and have made a number of 8"
long prospective labels from 1 cm half-round hardwood moulding. So far I
have painted the working surface white with a waterbased primer ..........."

2 possible answers,
1. Write on the label with a permanent black marker then put the label
upside down into the ground so the light cant fade the ink.
2. If the wood you have used is dark then write with either a white marker
pen or a chinagraph pencil.

I find that if the plant dies then the label stays legible for years, but if
the plant lives then the label fades inside a year, making me wonder if
there is something that the plants give off that helps to fade the ink.

Personally I use "bog standard" HB pencils on plastic labels.
--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk