Trees in most parts of the world are subjected to temperatures of 15 °F (-10 °C) and below in winter. Usually their roots are far too deep underground to freeze, and therefore these trees have no trouble coping with freezing temperatures at all. But with Bonsai, which are planted in shallow containers, the roots need additional protection in winter.
During the autumn, in colder areas, temperate trees start preparing for the coming winter by hardening up new growth and (for deciduous trees- ) dropping leaves to reduce moisture loss. This period of entering dormancy is important for Bonsai trees; don’t overprotect them by placing them inside. Over nurturing your trees will harm them over time. (Sub)tropical trees are the exception to this rule, as these should be placed indoors during the winter if temperatures drop below temperatures around 60 °F (15 °C). When placed indoors, tropical trees need lots of light and relatively high humidity. Only a spot immediately in front of a south-facing window will provide enough light.
What follows in this article is a description useful for most outdoor Bonsai (temperate species, including pines, junipers and maples), kept in climates with cold winters. But make sure to check our tree species guides for specific information!
Preparing your Bonsai trees for winter
As mentioned in the introduction, temperate trees need to be exposed to cold weather in fall in order to enter dormancy. For most tree species this means you should wait until the first frost before putting your trees in winter storage or adding protection.
Keeping your trees in a greenhouse or cold frame during the winter is recommended for those living in cold areas, where temperatures often drop below 15 °F (-10 °C). A cold frame helps to reduce fluctuations in temperature, and protects your trees from big temperature drops at night. It also prevents your trees from drying out from strong winds.
When such shelter is not available, you can place styrofoam covers around the pots to protect the roots, or plant your Bonsai including pot in your garden, covered with soil just over the roots. Make sure to place your trees at a spot without too much wind.
In milder climates, with temperatures of around 25 °F (-4 °C) at night, a cold frame won't be required. We still recommend to protect your trees from strong winds, and to place your trees on the ground. This helps to prevent sudden temperature drops. You might even cover your Bonsai's pots with a bed sheet or other insulation.
When dormant, be careful not to expose your trees to an extended period of high temperatures as this might bring them out of dormancy. Once a tree starts to grow it is very vulnerable to freezing temperatures, buds easily die - which significantly impacts the health of your Bonsai. To keep trees dormant throughout the winter, open up greenhouses when these warm up during sunny winter days. Once a tree is no longer dormant, protect it from any late frosts that might occur.
When overwintering, keep a close eye on your trees. Water only when the soil dries out; the trees don’t need much water when in dormancy so be careful not to water too often. Also check your trees for insects and infections regularly. During the spring you can place your trees outside again, but be alert to protect new growth against late frosts.