A common concern of bonsai hobbyists is what to do with their outdoor bonsai trees in areas with freezing temperatures in the winter months. Deciduous bonsai tree species need this winter dormancy so it is best to leave them outside to experience the seasonal cold.
However, unlike trees planted in the ground, bonsai trees are in containers. Being in containers their roots are less insulated and could freeze. If this occurs the tree could die.
To protect their bonsai trees experienced bonsai enthusiasts use overwintering techniques to bring their trees through the winter healthy and ready for spring.
The following sections describe the most common overwintering techniques.
One of the most preferred overwintering techniques is placing the bonsai, pot and all, inside a hole. The surrounding soil insulates the pot and the roots within from temperature extremes.
First, choose a location sheltered from wind. Optimum placement would be a location sheltered from wind and under the cover of a larger tree. Next, dig a hole deep enough so that the pot, when placed inside, will be slightly lower than the soil line.
After, place a humidity tray upside down into the bottom of the hole. Place the bonsai tree on top the ledge created by the tray.
Once completed, wet pine bark mulch should be filled in around the sides of the pot inside the hole. After, a top layer of pine bark mulch should be placed on the surface of the bonsai pot’s soil to make it even with the surrounding soil line. The pine bark serves a few purposes; it insulates, keeps the tree watered, and makes it easier to take out of the ground later.
Lastly, a plastic sheet can be placed over the bonsai as further protection if freezing precipitation is forecast.
The other option is to place the tree in an unheated room such as a shed, garage, or unheated greenhouse where the temperature won’t rise above 50 degrees Fahrenheit. If the temperature rises too much the tree may come out of winter dormancy too early. If this method is used, it helps the roots to keep the soil more dry than wet, but not completely dried out. Additionally, inspect the tree to make sure there are no lingering insects that may try to make a meal of your tree.