Bonsai care in winter is a serious concern. All container plants need special consideration during winter - bonsai are no exception.

There are many techniques to keep your bonsai safe. Where you live will make a big difference.
Will you need a greenhouse? Can they stay outside in the snow?
Can you use sheets of plastic to cover your trees?
As the old saying goes
"It all Depends!"
While traveling to give bonsai programs, I saw many resolutions to the winter problem -- amazing 'plant pits', cold houses, greenhouses and half-in-the-ground preparations.
However, winter in colder zones is not my field of expertise.
I have discovered many good articles on the subject and my suggested reading is Harry Harrington's excellent article on:
Harry is a well know British bonsai artist. Above are some of his bonsai under the snow in England.
Are you growing tropical bonsai plants outside of a warm climate?
When there is a "light frost" warning, the plant blankets mentioned below may work for you. If it is late in the season, it may be a little too risky.
Most hobbyists grow tropical bonsai outdoors in late spring through the summer and bring them indoors for the winter. See the page on growing bonsai trees indoors, which includes special lighting links.
If you have the space, a small greenhouse may be ideal.
If you live in the tropics, snow is not likely to be a problem. However, bonsai care in winter includes watching the weather for frost and freeze sneak attacks!

When the cold heads into tropical and subtropical regions, even a light frost for some tropical plants can be devastating.
Growers in warm climates, often use 'plant blankets' (as shown here). This lightweight fabric can add 2-8 degrees F to the temperature, depending upon the type you use.
This is usually just enough to do the job. They are available at many garden centers and bonsai shops.
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.
Plastic conducts cold and direct contact can cause great damage.
Also, remember to remove it when the temperature rises.
If your collection is small and you can carry your bonsai, bring them indoors for the best protection! A night or two indoors is much better than risking a frost or freeze.
Be sure to read the general Bonsai Care
page, and there's an interesting page on "winter silhouettes".
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