I'm using plates ,ceramic, as border edging

Hello Nancy. It would be best to spray the plates with a sealant. However, be sure to get the kind for outdoor use and one that is not penetrating like those used on pottery. Go with a surface sealant like Krylon Clear Polyurethane to protect your plates from the elements. Good luck!

apply a weather resistant finish on them first.dig the dishes in a few inches

In garden? No not really, they may crack in bade weather conditions, but most of the time they will be fine. (Think of all those bits of pottery dug up in ancient digs).........

It's not necessary but a layer of outdoor sealant might help it stand up to the weather