I generally use Sharpies. I’ve tried them on window blinds and other plastic. They did not last all that well, not for the season. I’ve cut a plastic chicken feed bag and wired that to my tomato cages, marked with a Sharpie, to tell varieties apart. I think that helps keep the birds away, with them flapping in the wind. I tried tying them with string but the wind unties them. The ink fades on them before the season is out too but it lasted long enough.
I’m OK with the markers only lasting a season. I always shuffle things around in the garden and I often grow different varieties of many things. I do have some stand-bys but a lot of the markers aren’t really reusable.
I take a 2x4 (I usually use treated but you can’t be certified organic if you do that. Sometimes I use a scrap 2x4 that’s not treated) and rip it into strips ¾” thick or thereabouts. I cut most of them into 18” lengths but for some stuff I use 24” lengths. I sand off the ends where I will mark them with the Sharpie to get it smooth. I can read those through the season but some can get a little faded. The next year I sand off where I had marked and re-mark them. Often I get two years out of them, sometimes more, sometimes less. You can get a bunch out of a 2x4.
If you want something to last longer and be easier to read you could paint them then mark or maybe mark and coat with urethane. Not sure how organic that would be. Or you can get a Dremel tool engraver and use metal or plastic. That should really last but it depends on the material you use how well you can read that engraving.
Lots of different ways to do this but I can’t say I’m overjoyed with the one I use. Still, it works for me.
