I have many plastic knives out in my garden. The sharpie wore off the first season. I leave the knives to mark where the plants are. But I'll be darned if I could tell you the names of the named plants any more. I've also used forks, tines down with names on the handles. If you do this and you want them for a longer time either on plastic ware or on mini-blinds, I suggest using a spray-on clear coat protection after writing the details.
Actually on things that you can use pencil on, the pencil outlasts a sharpie. I'm thinking the next time I want to use a plastic utensil, I will rough it up with sandpaper first and see if a pencil will write on the roughened surface. Then clear coat it too.
All plastic gets brittle after a couple of seasons too.