For a tag that will last for many years, cut up an old pie dish. You know, like a TV snack etc comes in made from heavy aluminium foil. Cut it into tag sizes and shapes and use a pencil to inscribe onto it. Then fix it to the
plant with a bit of wire or those twist ties etc. I've got some on
trees tbat are over 10 years old and you can still read them very well. If it's for something you can't tie the tag to or you can't nail it to, glue it to a regular type plastic tag or a popsicle stick and just poke it in the ground. The stick will rot away long before the alfoil does.