How Do I Raise A Flower Bed?

with existing plants already there? I don't want a gigantic one, just want to it to be taller and fuller if that makes sense.

I currently have two crepe myrtles, a gardenia, and a butterfly iris. I'm slowly adding plants little by little. Anyway, I want it to have some depth and "mounds" to show movement or give it more character. It is just two flat and boring right now. Can I just add more dirt around the plants that are already there and then mulch on top of that? Or do I have to dig the plants up and start over? The soil that I have now is fine.
How Do I Raise A Flower Bed?
For the established plants it wouldn't be a good idea to go much more than say 4 inches of new soil. One solution is to use rocks as big or bigger than bowling balls next to existing plants which gives the illusion of height while increasing soil height in unplanned areas. To really set off the rocks use several next to existing plants as a background then add low voltage lighting and now you really have something extra nice.