As the saying goes, "The best time to plant a tree was twenty years ago. The second-best time is now." Don't hesitate to plant a Japanese maple (Acer palmatum) if your climate can support it. Japanese maples are generally small and delicate, but you may plant this surprisingly hardy tree anytime the soil is not frozen. Certain times of year are better than others, however, and help ensure a healthy, attractive tree.
If possible, plant your Japanese maple in late fall, after the tree has dropped its leaves for the year. Not only does planting late in the season avoid summer heat, but it gives the maple a jump start on spring root growth, allowing the maple to produce a network of fibrous roots before the next summer's heat arrives. The dormant tree is also much less demanding of its environment, preventing leaf scorch and other problems.
If you choose to plant in spring, plant your Japanese maple as soon as you can work the soil. Dig the hole when soil thaws and plant the dormant tree. Don't plant in extremely wet soil, however, or the disturbed soil looses its texture and hardens into the garden's version of concrete. Maples break dormancy early in warm spring weather, and getting them into the ground as soon as possible increases their root network and their survivability.
Japanese maples do not like heat, drought and direct sun. Although you may plant container-grown Japanese maples any time of year, the summer heat often takes a toll on the tree. Scorched leaves and sunburned bark are not uncommon and stress the maple. If you must plant a maple in summer, water it deeply -- at least to a depth of 12 inches -- and mulch heavily. Protect it from full sun and drying winds.
Japanese maples develop an extensive network of interwoven roots, rather than having a single taproot. Transplanting Japanese maples from one site to another removes much of this network, and you risk losing the tree. Established maples do not respond well after digging, removal and replanting, but moving the tree during its dormant period increases the odds it will survive. Move as much of the root ball as possible and keep the soil surface level with its new location.