Did a little testing with my plant markers today. First, it would be no problem for the plant markers to be underground - they're tough and won't disintegrate.
And obviously they won't rust. You can write on them very easily with Sharpies. After I let the writing dry for an hour, I soaked in hot water for 8 hours then tried to rub it off. The writing didn't budge. However, I did scrub the writing with dish soap and hot water and it did smear a bit, but I doubt they would take that kind of abuse underground unless you've got some really dastardly earthworms. And yes, I have lots of them!
