Ornamental cabbage? Pansies? Other cold tolerant annuals? Purchased as six-packs, they might establish well enough to provide some winter show. Personally, I like the cover crop idea. Dahlias overwinter here in the ground in spite of the 50 inches of winter rain. It rarely drops below 25 F, and we haven't had measurable snow in two years. Kathy Stokmanis zone 8/9, where I have never planted ornamental cabbage but perhaps should try it before I recommend it to someone else.