Sharpie on white plastic knives. Buried next to the plant will be legible and durable for several seasons.
Soft lead pencil on vinyl mini blind will last several seasons above ground and still be legible. Write the name on the buried end also just to be sure.
Grease pencil/China pencil on mini blind/regular blind slats lasts several seasons and is readable from a distance. Works well with aluminum blinds. The blind slats can be cut to length with scissors.
