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No winter protection is needed for temperate climate woody plants until the temperature falls below 15F (-10C).
Below this point some kind of freeze protection is needed. I live in USDA Zone 8 and each year I prepare all of my container plant areas for freeze protection by programing the irrigation system to come on if I expect the low temperature to approach 15F. When water freezes it actually releases a good deal of heat. Of course the ice doesn't heat up, but what does happen is that the temperature of the ice does not fall until all this heat is released and radiated into the surroundings. In addition to this, the ice forming on the plant can also insulate it somewhat, protecting it from falling air temperatures. This form of frost protection is widespread in the orchards and vineyards of our area.
Tropical bonsai, in general, need protection from freezing. This means bringing them in the house in the winter or keeping them in a greenhouse (See the article Growing Bonsai Indoors ). Subtropical species which are tender but can tolerate some freezing must be protected from temperatures which will kill their small branches or roots. This varies by species and you must do some research to find out just how low a temperature they can tolerate. In general, these species have shallow or no dormancy requirements and can be brought into the house if a suitable environment can be established.
Other simple, temporary freeze protection measures include placing a tarp or plastic film over the plants for the night and removing it during the day, or moving the plants to an unheated, but protected area. These are methods for minimal freeze protection.
If you live in Zones colder than 8 and experience low winter temperatures below 15F, then you need more protection.
In areas of heavy snowfall, nature will do most of the work for you. Make sure that your plants are well watered before the freezes and snows hit. Before the earth freezes, heel your plants into the ground and cover the tops of the pots and lower stems with a mulch such as pine, cedar or fir bark. After the first snowfall, cover your plants with snow and make sure that they stay covered with snow all winter. It's as easy as that.
If you don't get a lot of snowfall, or you can't depend on a regular snowfall, then other measures must be taken. One method is to build a cold frame to house your plants in the winter. There are many plans for cold frames and all of them will work as long as they keep the temperature from falling below 15F. I strongly recommend that you place a minimum-maximum thermometer in the cold frame to monitor the temperature. Cold frames do have one serious disadvantage, they can heat up if the sun can shine through a transparent housing. For this reason it is best to place it out of direct sunlight or construct the covering with translucent, not transparent materials. You do not want the temperature to rise above 40F for any appreciable period of time. This you can monitor with your min-max thermometer. In very cold areas, heating cables can be installed in the floor of the cold frame.
Another method of freeze protection is to house your plants in an unheated garage, basement or other structure for the winter. Again, it should be monitored to make sure that the temperature does not fall below 15F. Small space heaters can be installed with a good low reading thermostat to heat the enclosure when the temperature starts to fall below 20F. High temperatures should not rise above 40F for more than a few days at a time.
A more bizarre, but perfectly acceptable method of over wintering is to keep your plants in the refrigerator during their dormant period. This works well if you only have a few plants and you need to give them the required dormant period and plan to return them to an indoor environment in late winter. The constant 35 to 40F temperature of most refrigerators is ideal for winter storage.
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