Trees generally can be separated into three different groups: hardy species (can tolerate freezing), subtropical species (withstand cold, but not freezing) and tropical species (warm environment at all times).
An interesting tree care point to note is that hardy and sometimes subtropical species require a dormant period within their yearly cycle, without it they can become sick and even die over time.
When choosing a bonsai it is important to consider the types of environments you can create for it within/outside your home otherwise it will struggle if not given the required temperatures.
For example subtropical trees are used to warm summers but cooler winters, when outside they like sheltered areas, while indoors they like cool locations during winter months.
Use the useful diagram on the left from The Bonsai Handbook to help you decide what temperature will suit your bonsai tree species.
If you have an indoor bonsai, it is a good idea to move it around the house to provide the necessary temperature requirements for your tree to thrive, whether it be close to a window to receive plenty of light in the summer or in a dark hallway/under the stairs to keep it cool.
Think of dormancy as a survival strategy for trees to remain alive during winter. Growth is reduced and soft tissue is prepared to make a temperate climate species less susceptible to the harsh freezing conditions.
A bonsai tree’s biological clock will either need to be honored or else slowly manipulated by being brought inside after receiving at least 1000 hours of colder temperatures below 50-40°F (10-4°C). The tree will then begin growing again, however be careful to not bring it inside to an overly warm environment.
To induce your bonsai tree into a dormant state, they require shorter days and drop in the average temperature. Some species may require a period of subzero temperatures to become dormant. But refer to your trees natural habitat to decide the best temperature conditions.
Tropical and even most subtropical trees generally do not have a dormant period and do well in normal room temperatures in modern homes throughout Autumn, Winter and Spring.
If you have an outdoors bonsai in cold conditions, generally speaking it is best to try and protect the roots from freezing rather than protecting their tops.
Keeping it cool
Tropical and sub-tropical bonsai species thrive in warm conditions but preventing the pot from overheating will greatly reduce the risk of the roots burning.
Similarly, hardy trees can suffer from heat stress and will shut down if exposed to constant temperatures of above 86°F (30°C). So keeping the pot cool is a must-do (you can lay tiles over your bonsai pot which can help keep the roots cool – right image), providing well-ventilated shade and regularly spraying the bonsai trees foliage will keep your tree happy.