After much searching, this is my favorite marker to use on plant labels (wood, plastic, or metal). It lasts quite a while in the garden, much longer that a sharpie. Fine pointed felt-tip marker with an earth-friendly, xylene-free, jet black ink. Permanent on wood or plastic.
Available in fine point (.8mm) or regular (1.2mm). The fine point is perfect for the small wooden plant tags, the regular point is perfect for the 10″ wooden plant tags. I source these from a company that’s right up the road in Vassalboro, ME. Buy two or more and get them for $5/each (can be any combo of fine or regular point).