Hi Debbie, this seems to be a subject few practice. With the plants being exposed to the cold, I would imagine they would have to be of a zone at least 2 below the one you are in. Also the container would have to be insulated with styrofoam or like material. Would love to hear from someone with experience on this subject. I think many would try it if they had more knowledge.

Are you talking sun or shade? You can put pansies and ornamental cabbage and kale in pots outdoors in full sun. You can also use snapdragons, dianthus, primrose, and cyclamen, but you might need to walk your containers in and out if we have temps below 30 or so. The cabbage and kale, dianthus, primrose and cyclamen will tolerate some shade, but they ought to get at least 4 hours of sun to keep them blooming and colorful.

If you want trees in containers, you can use Japanese Maple and Taxus in sun or shade for winter, shade in summer, and Junipers in the sun year round in pots outdoors in your zone. Just remember, sometimes the pots themselves have trouble with sub-freezing temperatures.

Sally

I have a passion for thyme which remains evergreen throughout the winter months. There are a large number of thymes with different habits. Some are mat forming, creeping, upright. There's wooly thyme which is tiny and creeping. Then there's golden thyme, which is almost lime green in summer and I have one that looks more like needle evergreen. I don't grow them in pots but I don't see why you couldn't.

I would also recommend sedums, they do well in pots and like thyme, not very demanding.
Some dianthus stays green most of the winter and flowers through several frosts.

How about little boxwoods? And pansies and parsley.

I could think of more....Have fun.~

Would Hellebores and snowdrops take the freezing in a window box that's only 8 inches deep? True, it's not much for the early winter, but would seem to look nice poking up through some evergreen boughs.

I keep dwarf or slow-growing conifers (such as dwarf Alberta spruce, various creeping junipers, bird's nest spruce) in containers of varying sizes -- using the molded foam planters that look like terracotta. In stone (granite) troughs I have plantings of hens-'n'-chicks, dwarf cedars and cypresses, and cold-hardy sedums that drip over the sides.