The other problem with the plastic markers is that they're easily lost. They inadventantly get raked up, dug up, relocated by animals or "helpful" humans. What I've found is the thicker, less rigid types of plastic markers work the best. The thin, stiff ones will degrade in a couple of years. I've got plenty of plastic shards around the property.
Maps and photographic records are necessary when you have a lot of varieties.
