hmm, not quite sure what you have in mind. Do you mean for the letters to stand up 3 diamentionally? or did you want them set down low to the ground, with the letters facing upwards?

In anycase, the easiest plant I can think of is boxwood. It's slow in growing but so easy to shape and managed. There are other plants as well like yews that have a deep green look to them. Both shrubs are evergreens, so they won't drop their leaves in winter.

Ianna