Years ago I bought 'paint pens' from a welding supply house. We called them pens, but they're actually a tube of paint with a ball point tip, designed for marking on structural steel. The ball was about 1/16" in diameter, so fine detail is out of the question, but the paint is very durable.
Plastic degrades quickly, but I have had great success with vinyl plant labels. I mark the pointed, underground end with Sharpie, and the above ground end with #2 pencil. I have some that are 10 years old and still serviceable. I erase them with #400 Wet-or-Dry sandpaper for re-use.
Vinyl mini blinds work ok, but for some reason the soil eventually stains them to the point where they're almost unreadable.
I tried aluminum mini blinds, but they corroded to powder after about a year of soil contact. Fine, if you don't mind them getting shorter every year.
