Like most Bonsai techniques, how you overwinter your trees depends on the region in which you live and the tree species you have.

During the autumn, in colder areas, hardy 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 dormancy is important for Bonsai trees, so don’t overprotect them by placing them inside.

(Sub)tropical trees are a different story, these should be placed indoors during the winter if temperatures drop below temperatures around 60 °F (15 degrees Celsius) but can remain outside year-round in warm climates. 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, kept in climates with cold winters. Check our tree species guides for specific information per tree species however!

 

Protecting your Bonsai trees in winter

Trees in most parts of the world are subjected to temperatures of 15 °F (minus 10 degrees Celsius) 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. With Bonsai however, being planted in shallow containers, the roots need additional protection in winter.

Keeping your trees in a greenhouse or cold frame during the winter is recommended for those living in cold areas. 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 until the first branching.

When in dormancy, 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.

 

Japanese Zelkova Bonsai

A beautiful Japanese Zelkova Bonsai tree in winter, the ramification is incredible!

 

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.