Now that it is warm in Atlanta, I've started thinking about the coming fall and winter, and what I'm going to do with my windowboxes. Right now I am planning on autumn crocus and colchicum for the fall, but haven't made any plans for the winter. I'm trying to avoid the usual suspects (pansies and such) just for a challenge. I'm considering silk flowers in all sorts of unrealistic colors for the fun aspect. But I'd love any suggestions you have to offer! Here are the stats:
Winters are relatively mild; lows briefly in the teens, but I have no protection from the elements by ways of trees or such. We are subject to ice, but rarely snow. And while fall can be very dry, winters are almost always damp. Temperatures start to drop (consistently below 65F) and stay dropped in late October and November. January and February are the cold months.
Windowbox 1: located between two townhouses, facing vaguely southwest. begins to receive afternoon sun at 1 PM, and continues to receive sun until a couple hours before sunset. so, assume six hours of sunlight. growing space is 22" by 8", average depth 6".
Windowbox 2: located on the back of my house, facing vaguely northwest. begins to receive sun around 3 PM, and receives DIRECT afternoon sun till sunset. so, assume six hours of direct sun. growing space is 20" by 7", average depth 6".
Since I'm in zone 5, I can't have live plants in my window boxes over winter. The only thing I can think of that may survive over your winter is ornamental Kale. I read it will keep to temps as low as 5 degrees. http://www.thegardenhelper.com/kale.html
Up here in the north I make a dried arrangement for my boxes. I cut branches off of every evergreen I can find, redtwig dogwood and corkscrew willow branches, and dried hydrangea flowers. I stick the branches/stems well into the soil. Sometimes I have to use landscape fabric pins to secure the branches. It looks especially nice at Christmas time Everything stays green well into March. You may not get the same results in your warmer climate, but you could consider some kind of dried arrangement instead of live plants.
Hopefully somebody from your neck of the woods will reply with their ideas.