Japanese maples are very popular bonsai trees. Although bonsai trees are traditionally kept outdoors to allow them a natural life cycle, including dropping leaves in the winter, some cultivars of bonsai are suited for indoor cultivation in climates where the tree and rootstock may not survive well outdoors.
Japanese maples are, depending on the variety, are cold hardy down to United States Department of Agriculture Hardiness Zones 5b through 8. In cooler zones, like five, six and seven, you should check the hardiness of your cultivar before trying to winter over your bonsai outdoors. However, in USDA Hardiness Zone 8 or warmer, you can winter over your Japanese maple bonsai outside.
If you live in a colder climate or are concerned about unusually cold winter or time, a cold frame attached to your house can take the edge off winter temperatures. By making a lean-to style cold frame on the outside of your house, the heat from your house will raise the temperature inside the frame above the natural outdoor temperatures. This often will allow your maple tree to winter over and drop leaves without the risk of root freezing.
The coldest temperatures your bonsai will survive will depend on the variety. However, allowing your tree to remain below 40 degrees for several months is ideal for tree health. Some Japanese maples will survive winter temperatures as low as -20 degrees F. However, those maples are often planted in the ground. The soil temperatures below a foot or two are often warmer than the ambient air temperatures. Bonsai, because it grows in small pots, can suffer from root freeze very easily. If you are unsure of the hardiness of your tree, keep the tree in a cold frame to prevent the soil from freezing.
Do not water your Japanese maple bonsai very much during the winter. Water the tree thoroughly as the last few leaves drop off. Once it is leafless, water the tree once every week or two. If you have a very cold hardy tree that has roots that can survive below freezing temperatures, do not water your tree over the winter to prevent root freeze.
Pruning and Wiring
Prune and wire your tree in the spring as the tree begins to bud. Change the overall shape of the tree in the early spring. If you are going to use leaf-pruning techniques to reduce leaf size, perform your leaf pruning in the late spring. Wire the tree in early spring, and re-wire in summer to prevent bark damage due to the tree growing around the wire.
If you are keeping your Japanese maple bonsai as a primarily indoor tree, set your tree outside in the late winter or early spring to allow it a period of dormancy. If you cannot, consider refrigerating your tree until it drops its leaves. However, many varieties of Japanese maple bonsai will grow as an indoor only plant. However, they will be healthier if allowed to go through their natural fall and winter processes.