Bonsai care in winter is a serious concern. All container plants need special consideration during winter - bonsai included.
There are many techniques to keep your bonsai safe. Where you live and the type of bonsai you have, will make a big difference.
There are many resolutions to the winter bonsai care problems ... including 'plant pits,' cold houses, greenhouses and half-in-the-ground preparations.
Harry Harrington is a well know British bonsai artist. These are silhouettes of some of his bonsai under the snow in England.
Harry's excellent article on bonsai care in winter is highly recommended reading. He begins with "Most species of bonsai and nearly all temperate climate woody plants require the cold."
Later he adds "Some bonsai beginners feel that their trees may perish if subject to the harsh conditions of the winter months and bring their trees indoors to 'protect' them. This continuation of heat and light through the winter prevents dormancy in temperate trees. The resulting continual growth throughout the year goes against the trees' internal clock ..." This is important, read on!
Are you growing tropical bonsai plants outside of a warm climate? Most hobbyists, living in less than tropical climates, grow tropical trees outdoors in late spring through the summer and bring them indoors for the winter.
If you have the space, a basement set-up under lights will work. A small greenhouse may be ideal. See the page on growing bonsai trees indoors.
When the cold heads into tropical and subtropical regions, even a light frost for some tropical plants can be devastating. When there is a "light frost" warning, plant blankets may be all you need. Growers often use 'plant blankets' (as shown here) for frost threats.
This lightweight fabric can add 2-8 degrees to the temperature. The type that comes in a roll, is easy to cut to size and is especially popular. (It's also reusable.)
If your tropical bonsai collection is small, carry them indoors for the best protection. A night or two indoors is much better than risking a bad frost or solid freeze.
Covering your trees with plastic can help. HOWEVER, when using any type of plastic, always protect your trees with a layer of cloth or even newspaper between the plastic cover and your bonsai tree. (This also applies to small yard trees and shrubs.)
Plastic conducts cold and direct contact can cause damage. Also, remember to remove plastic when the sun rises. The plastic can "cook" your trees as well as protect them from cold.