Obviously, if you live in the tropics, the answer to this question is "never." Tender trees, or bonsai that are tropical or semi-tropical, must be protected from the cold. Hardy trees, or winter-needing bonsai, on the other hand, must be exposed to winter conditions.
Tender tropical and sub-tropical bonsai should be brought indoors before freezing
This is even before cool temperatures set in, to ensure the best continued growth. They require adequate light, humidity, and air movement for their health and maintenance. Tender trees may not thrive in these conditions, but they can be sustained until spring returns outside.
For hardy bonsai it's important to determine how hardy it is for your particular climate. If it grows in the ground in an area with winters more severe than yours, it will probably survived potted life outside in your area; if it survives winter in areas slightly warmer than yours, it probably needs winter shelter.
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If your tree is not likely to be hardy enough for your climate, you can follow our advice up to the point at which temperatures plummet and winter becomes severe. At that point bring your bonsai indoors, preferably into a cool area, like a garage or basement, until the worst extremes of winter pass. Return them to their outdoor sheltered spots as soon as temperatures return to reasonable. This "half-way" dormancy treatment achieves a happy medium and protects from the extremes of winter.
However, do not keep your outdoor trees inside for a long period. You want to protect it from harsh winter elements but at the same time maintain winter dormancy.
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