Thinking some more on this, I used to have a sign and silk screen business. I started out, as a kid in high school, doing some for local businesses. In the beginning, everything was hand painted. Later, when I got some jobs where the customer wanted hundreds of the same sign, we got into doing them by silk screen. (mostly real estate, political, and traffic signs) Along the line, we also, mainly, switched from painting individual signs to computer generated pressure sensitive vinyl letters.
These would be good for plant markers. They come in sizes ½ inch and up, in over 100 different fonts, and in any color you want. They are pre-spaced; you just peel off backing, and stick them on. They last for several years.
As an alternative, you could print the markers from your computer, with a laser printer. You would want to use a laser printer, because laser printing can take the rain; ink jet runs when it gets wet. You also want to print on pressure sensitive polyester, or mylar. Paper deteriorates in the rain, and vinyl doesn't do well with the heat in the laser printer.
The vinyl letters, or the mylar can be stuck on aluminum, or corrugated plastic poster board, etc. We use sheets of little mylar labels to make plant tags, that can be stuck on the side of pots, or on pastic stick-in-pot tags. I know, though, that it comes in larger sizes, like full 8½ X 11 sheets
Here is a vinyl letter supplier.
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