In open ground, the roots of plants aren't confined to a small space. Rain and snow melt moisten the soil relatively evenly and it remains moist for awhile because evaporation slows down in colder weather. When the ground freezes it does so in a uniform fashion. During an especially dry winter you may need to water your garden beds occasionally, but most perennial plants are adapted to handle harsh conditions.
The soil in pots can partially thaw on a warm day and refreeze when the sun goes down. Any moisture from snow will drain or evaporate more quickly than a standard garden bed. These changes in the soil can stress a plant to death. With a little extra attention you can help moderate conditions by recognizing these changes and acting to save your potted plants.
Plants don't need much water during winter. They aren't typically growing during the cold; they're dormant. They're alive, but nutrients aren't flowing from the roots and photosynthetic sugars aren't replenishing energy. Water isn't needed to sustain this process. Instead, water is primarily needed to keep the roots from desiccating, from drying out.
|My herb garden.|
I grow many of my herbs in pots. Most of them are annuals, but the rosemary, oregano, tarragon, and mint return every year. My thyme stays viable most of the winter; I can snip off usable portions in December. After prolonged periods of dry weather I test the soil and water if needed.
How do I test the soil? By sticking my finger in it. If it's frozen and I can't push in more than an inch, it doesn't need water. If my finger penetrates easily and the soil is dry, it's time to water. You won't need as much water in winter as during the growing season. You only need enough to moisten the soil, not soak it.
You won't need to water as often as during warm weather. Every couple weeks or even just once a month may be enough. When the soil is frozen you shouldn't water, so it's only during the time when the sun comes out and begins a thaw that you need to think about it. You're just trying to avoid long, sustained periods of dry soil.
Too much water in a pot can have devastating results. Remember that water expands when it freezes. Also remember that a pot is a confined space. If the soil is saturated when it freezes, it can break the confines of the pot. I've lost a few Terra Cotta pots in years past because I watered too much before a freeze. I use plastic pots now for many of my plantings, particularly herbs.
|The pots that broke during a freeze.|
If you have pots exclusively for annuals, you don't need to water them during the winter. The soil will dry out and you'll need to spend extra effort in the spring getting the moisture content up before planting, but you won't have to worry about pots breaking during the cold.
|I don't water these pots; they're used for annuals.|
It's all very easy. Every now and then stick your finger in a pot. If it's dry stick your finger in another pot. If it's dry too, pull out your watering can and spend a few seconds pouring water into the pots with perennials in them. Do this a couple times during the winter and your loss due to winterkill should be reduced.