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.
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 Bonsai Care in Winter
Harry is a well know British bonsai artist. These are silhouettes of 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 (as 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 does include watching the weather for frost and freeze sneak attacks that occur primarily in the sub-tropics. (Such as South Florida.)
When I lived In South Florida there were occasional freezes.
The farms and nurseries (including mine) ran the irrigation systems all night to keep the plants from freezing solid.
As counter-intuitive as it seems, the ice protected the plants.
No matter how cold it got, the plants never went below 32 degrees F
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) for frost threats.
This lightweight fabric can add 2-8 degrees 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.
(This also applies to small yard trees and shrubs.) Plastic conducts cold and direct contact can cause great damage. Also, remember to remove it when the temperature rises.
If your tropical bonsai collection is small and you can carry them, bring them indoors for the best protection! A night or two indoors is much better than risking a bad frost or solid freeze.
Be sure to read the general Bonsai Care page
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Know the basics? Ready for more?
Watch these amazing Colin Lewis bonsai class videos