I saw one garden that had the plants all marked with clay flower pots. Some 4", but most were the 6" size. The pots were inverted , and placed in front of the plant. The "gardener had painted ( quite beautifully!) the name of the plant / hybridizer, etc. ,on the pot, with a "pearl "colored paint, that was sort of "metal flake" ( I'm sure one of those "paint pens" would work
), and the pot, when inverted in it's 'place', covered a metal tag, staked to the ground with about 3"-4" of "stake" sticking out of the ground......a "second line of defense" against 'losing" a cultivar, and, it helped keep the pots where they belonged. They had only a hundred or so, iris cultivars, but probably another 200 "other" plants, all marked this way. It looked very nice, but, it wouldn't be cheap !