To properly cultivate and maintain most bonsai trees they should not be kept inside but should be outside year round.
There are a group of what are called indoor bonsai trees.
These are from warmer climates and are usually non-traditional species such as palm trees.
Because they are from a warmer climate they do need to be kept indoors in most locales.
However if you are keeping traditional native bonsai trees in the US then chances are that tree needs to be outside.
Most people do not realize how important it is for these trees to be wintered outside in a natural climate.
For your tree to have a healthy and natural growing cycle it is critical for it to have this period of winter dormancy.
To give your plant this necessary period you will need to winter it someplace outside.
The best way to accomplish this is to remove the tree from it's container and plant it in your yard.
In most native trees the roots are the part of the tree that is most sensitive to the cold.
Most plants will start experiencing root damage at about 23 degrees Fahrenheit.
To help avoid root damage you will need to take the plant out of its container and then bury it in the yard or garden.
If you bury the plant so that the bottom branches are just barely exposed this should help keep the roots below the frost line.
You should bury it in a shaded area.
During the winter days the sun can cause the plant to lose it's water and the frozen roots will not be able to replenish it.
If you do not have a yard to plant the tree in for the winter you can use a large container.
A large container about the size of a vegetable crate should be sufficient.
You bury the plant in the container just like you would in the yard.
And set it somewhere outside where will be exposed to natural elements.
If you use a container it is very important that you monitor the soil moisture closely and do not allow it to dry out.
You can also winter your tree in an attic or garage as long as it is not heated and has windows.
Be sure to place the plant where it is exposed to the light from the windows.
You will still need to water the plant almost daily if you choose this method.
If you plant is a conifer you will want to rotate it about once a week to so that all sides of the plant will be exposed to the sunlight.
You can prepare your tree for it's winter storage as soon as the last leaves have fallen off.
At that time the plant is dormant and ready to be moved.
In the spring you can remove it from the yard and place it back in a container as soon as you see the first buds appear.
If you allow your plant to winter indoors they will bud early.
When they bud early the days will still be too short for proper growth and the leaves will have long gaps between them.