Answer: Although an "ideal" time to transplant trees is in fall, when there is no green growth but the roots are still active. This allows the tree to put all its efforts on establishing the root system. However, given proper transplanting care, a tree can be moved at any time. Make sure the hole is adequately dug, adding lots of compost or other organic matter. Japanese Maples like evenly moist, acidic, and well-drained soil. A handful (according to package directions!) of super phosphate will accelerate root growth. Water the hole before putting the tree in, and then water well again after the tree is set. The critical care is the continued watering. Transplants should be carefully monitored and receive sufficient water on a regular basis throughout the entire growing season.
Japanese Maples can be grown in Zones 5-9. They can take full sun in the cooler climates, but prefer part shade in the warmer ones. In Zone 5 it is generally recommended to plant them in a more protected area rather than in the open for winter protection. In your zone 6, your Japanese Maple should do fine in a little more open area. However, as you noted how well it was doing in the shade of the dawn redwoods, consider the possibility of adding a larger growing tree as well to add a bit of shade.
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