When growing bonsai, we aim to set ourselves up for success by doing proper research. I see many people who buy trees that do not grow well in their climate and these trees inevitably die or struggle. To avoid this, becoming familiar with zoning and how to use it, is essential. The USDA has a useful zoning map that categorizes areas based on freezing temperatures. Below is a photo of the USDA zoning map.
This map is especially important if you live in a climate with freezing temperatures. Many trees will not tolerate certain thresholds of cold and this map can help identify those thresholds for your tree. You must do some research to find out which zone your tree is classified under. For example, a Ficus carica can tolerate zones 8-10. Using the chart, we can see zone 8 has a minimum freezing temperature of 10-15 degrees and zone 10 has a minimum of 30-40 degrees. This information means a Ficus carica can tolerate low temperatures from 10-40 degrees. Even if your tree can survive freezing temperatures, it is wise to use a greenhouse. Freezing temperatures tend to kill fine branching on refined bonsai so a greenhouse acts as a buffer against this damage. It is recommended to keep your tree in the greenhouse from winter to early spring to prevent frost damage. This hardiness map has its limitations because it does not consider heat intensity and humidity of different zones. An excellent source for determining heat tolerance is the heat zoning map made by the American Horticultural Society. This map uses the average number of days per year above 86 degrees Fahrenheit as the measuring standard. If you do not know the heat zoning of your plant, you can look it up online and then use this map below as a reference.
American Horticultural Society. (n.d.). https://ahsgardening.org/gardening-resources/gardening-maps/.
United States Department of Agriculture. USDA Plant Hardiness Zone Map. (n.d.). https://planthardiness.ars.usda.gov/PHZMWeb/.