Q: So what should I do with my cold-weather bonsai trees in the winter?
A: There are many options here. You should begin by choosing a location where the temperature is unlikely to drop below twenty degrees. If all you have available is an unheated porch or a garage that could possibly get colder than that, you may need to insulate your trees further. Nor should the high temperature during the winter months be higher than fifty degrees Fahrenheit for more than a few days in a row.
If you have settled on a location where the average temperature will stay between thirty and forty degrees, the trees, even the evergreens, will no longer need light. But beware: the warmer they get, the more light they need!
Temperatures of fifty degrees Fahrenheit are considered temperate; a tree kept under these conditions would need light to survive the winter, as dormancy would not be total.
It is also possible to put your trees into a box of mulch or newspapers to keep them more insulated. Small collections can always be moved temporarily to a more sheltered spot, to the basement for example, if the weather is really cold for an extended period of time (say, a week or so), and then they can be brought back out to the garage when the danger is past.
Cold hardy trees can be stored in a cold cellar (below 35 degrees), an unheated garage, cold frame, or covered trench. On warm days, check to see if watering is needed. These trees can be wintered in darkness, but need as much light as possible when they start pushing in the spring.
Q: Is the procedure the same for deciduous trees as well as evergreens?
A: Basically, yes. Once the leaves are gone, photosynthesis no longer takes place, and a deciduous tree is not going to need light no matter what the temperature is.
Not until dormancy is broken and the new leaves start to open in the Spring does the need for light return. Therefore, it is important to make sure that dormancy isn’t broken too early in the year. If that happens, the leaves open early, too early for the tree to survive outside, and you have to find a suitable place for it inside.
You will know that you have found the perfect spot for wintering your cold-weather tree if the seasonal changes that occur with your Bonsai mirror the changes actually taking place in nature.
Trees that overwinter in a basement will usually have conditions somewhat warmer from the outside ones, the leaves will open up earlier, and then you’ll have to find a sunny spot or supplement light in some way, since the new leaves need bright sun and longer daylight hours to grow.