Answer: This sounds like quite a gardening experiment! Ideally, you can raise the bed to a depth of at least 18 inches -- I'm not sure how well shrubs will fare in less soil during the winter. If you can't practically raise the sides of the bed that far, then at least mound the soil high in the center and grade it to gently slope to the edges of the bed, and plant shrubs in the center. Have the soil tested for pH and nutrient levels, and adjust those as needed to ensure healhty growth.
Potentilla, blueberries (which have beautiful fall foliage as well as being fruitful), barberry, dwarf azaleas and rhodies, and dwarf conifers (pines, junipers, arborvitae, spruce) are some options to consider. These plants will need regular watering, since they have such a limited area in which to seek out moisture. Mulch them well, and water them until the soil freezes, and wrap the shrubs for the winter to prevent the branches from drying out.
I am a little concerned that the soil volume will be enough to protect the plants' roots for the winter. I suggest that you plant perennials no closer than 8" from the perimeter, and use the perimeter for annuals only. The extra buffer of soil on the edge will help protect the perennials' roots. I hope this helps!
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