I just saw a memorial garden that sounds similar to what you are describing. They used large granite stones/small boulders and had the names engraved into the stones. I imagine you could have that done at any monument company.

We had a speaker at our meeting last night who suggested that the memorial/donor plaques not be attached to the individual plants in case the plant died. Her group felt that it could potentially be devastating if a plant donated in memory of deceased Grandma, whoever, became deceased itself.

I don't know if I hold w/this theory - a plant is a plant, they can be replaced!

Our memorial gardens at church are not labeled outside, but rather a diagram is kept in a hallway near the windows that look out. The garden then continues to look like a garden (rather than a species planting, with labeling), but each plant is still labeled with pertinent information.

Snowgardener had a good idea. I was going to suggest something similar... sandblasting that info into a large stone. But, that can get expensive unless you have a MG who works in a monument company. We are fortunate here in that case. But even that would tap him too much for his generosity.

Why not consider painting a sign. Aerial view. Plants numbered on the layout with a legend below along with the donors. Surely there's an artist in your group who would help with that.

Just an idea.
Xtal

ah- now I would have to suggest copper flashing - get someone with a nice hand to write the information on a section of flashing, and it makes a lovely surface to emboss (well, chase, since you're working from the front)...but then the 'hard to remove' came up, and all I can think of are those motion-sensitive sprinkler heads, to keep the deer off your daisies? that's a lot gentler than a volunteer with a shotgun and rock salt....and still gets the point across.