The handsome branching structure and ornate leaves of Japanese maples (Acer palmatum) make them prized additions to a landscape. The species Acer japonicum -- more often called fullmoon maple -- may also be called Japanese maple. Regardless of species or cultivar of either, the trees grow well in containers that are large enough to sustain the growing root systems. They survive winter outdoors in containers as long as the penetrating cold does not reach a point that can kill stem and root tissues.
Japanese maple trees can tolerate winter cold better when planted in the ground. The soil insulates the roots, diminishing fluctuations or severe drops in winter temperatures. Cultivars of Japanese maples may demonstrate varying winter cold hardiness. A general guideline is that Acer palmatum survives winters in U.S. Department of Agriculture plant hardiness zones 6 through 8, with temperatures as low as minus 10 degrees Fahrenheit. Acer japonicum survives winters in zones 5 through 7, where the lowest winter temperature may be around minus -6.67 degrees C.
Trees in Containers
Even though the tree is dormant in winter, the soil must remain evenly moist. Watering is necessary as long as the soil is not frozen. Container soil becomes much colder than the ground soil since the container walls are surrounded by cold air. This causes cold to penetrate more easily to the roots, harming or killing them. Roots of Acer palmatum may be killed in soil that drops to 14 degrees.
In some regions, Japanese maples may survive the winter outdoors with no problems. Whatever the USDA plant hardiness zone rating is in your area, consider it one zone colder if growing a maple in an above-ground container. For example, Acer palmatum is hardy in the ground in USDA zones 6 through 8. But when grown in containers, a zone 6 location is similar to zone 5 because of the colder air that penetrates the container soil and roots. Only in zones 7 and 8 is it safe to leave a cultivar of Acer palmatum outdoors over winter in a container. Likewise, for Acer japonicum, it's safe for containerised trees to stay outdoors only in zones 6 and 7.
It's possible to overwinter Japanese maples in containers if precautionary steps are taken to insulate the root ball. Avoid windswept areas in winter when choosing a location. Digging a hole in fall and sinking the container into it provides protection to the roots. Lift the container and tree in spring. Wrap old blankets, fibreglass padding or styrofoam around the container if left above ground. Placing straw bales around the container and filling empty spaces with loose mulch or straw also can diminish the effect of chilling from cold winter air temperatures. Lastly, the Japanese maple may be moved indoors into an unheated outbuilding near a window to safely overwinter.