Just wondering if anyone has any suggestions for how to deal with an unusual garden bed. I was all set to plant a nice shade garden with hosta, ferns, coral bells, Solomon's Seal, Bleeding Heart and Jacob's Ladder until I troweled a little today and discovered this bizarre construction installed by a previous owner of our house.

Here's how the bed is constructed:

LAYER 1: The entire bed is filled with medium sized rocks about six inches down.

LAYER 2: Layer of small rocks.

LAYER 3: On top of the small rock layer is a heavy black plastic mat.

LAYER 4: 1 to 2 inch layer of soil.

LAYER 5: (Ground level) Currently the bed is filled with happily growing Sedum sarmentosum interspersed with small beach stones.

I assume this was constructed to prevent weeds and reduce the work load for the previous gardener.

So now I'm thinking "rock garden". I don't really want to embark upon the massive project of digging out all the rocks. Is there anything that will grow in these shallow soil conditions other than Sedum sarmentosum ? I like the sedum very much, but I was hoping to supplement it with some flowering shade perennials, or something with interesting leaves, preferably native to the northeast of the U.S.A., or cultivated versions of northeast natives.

ZONE: 6B.

LIGHT: Partial shade.

SOIL: Well-drained to dry-ish. Seems drier than the area next to it. The area next to it (without rocks) is completely carpeted with spongy moss.

Any suggestions would be appreciated. Thanks!