Ordinarily, sitting in my un-airconditioned room on one of the hottest days of one of the hottest summers I can remember, wouldn't inspire me to write an article on how to overwinter perennials in containers. But I couldn't ignore the wonderful folks who posted pleas for advice on what to do with plants in winter. (And who knows, maybe the "think cold thoughts" strategy will actually do some good :). So, I'll give it a shot.
Everything I've ever read about growing perennials in containers beyond their zone recommends bringing the plants inside to a greenhouse or sun room to protect them from the cold.
If you are lucky enough to have enough room and can bring all of your plants indoors, don't be worried if they seem a little stressed and drop their leaves or flowers. Many plants go into a state of dormancy over the winter. The best strategy for keeping them happy is to pretend that the goal is to just barely keep them alive... water sparingly, don't fertilize or prune. Keep them in an area where the temperature stays relatively constant, and mist them regularly, or set them on a tray of gravel and water to increase the humidity if your house gets very dry in the winter.
Most tender perennials (e.g. geraniums), succulents, and herbs should be moved inside. If you don't have enough space for the entire plant, you should at least take a few cuttings to overwinter inside, to make sure you don't lose the whole plant if it's a severe winter. The cuttings should be stored in a light, dry, frost-free area. They can be planted again in the spring when the weather warms.
A lot of hardy perennials can survive just fine outdoors, or in an unheated space, as long as you protect them from severe cold.
There are several issues that plants face in winter, that you'll need to deal with if you want to leave the plants outside...
- frost damaging leaves and tender shoots
- hard freeze killing the entire plant
- freeze/thaw action of soil disturbing the roots or cracking the container
- weight of snow or ice breaking the plant
The goal of overwintering most hardy perennials is to prevent them from getting too cold - not to keep them warm. It is winter, after all. A good winter temperature range for most plants is between 32F and 45F.
The most common approach for protecting plants outside is to wrap your pots with some kind of hay and burlap to insulate them. But if hay is too messy, there are lots of other things you can use... leaves, blankets, bubble wrap, those annoying styrofoam packing peanuts no one can ever get rid of. Any insulating material should do the trick. I've heard of folks setting pots into a big cardboard box full of styrofoam peanuts and covering the top with a thick layer of leaves. Remember, your goal is just to keep the pot from freezing - not to keep it warm.
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