Answer: Midsummer is not a good time, so you should wait until next spring if you can. Although many plants can be transplanted successfully in the fall, these are very shallow rooted and in my experience do better if planted in the spring. Late winter to very early spring would probably be the best time to do this, while they are still dormant and before they begin to grow for the season. It is better to move them while they are small so the process is less disruptive especially since you may have to bare root them. (Ideally you would dig them with a trowel when they are only a couple of inches tall -- soon after germination.) Waiting until spring will also allow you to see if their fall color is adequate, as this may vary from plant to plant. You also want to select for overall shape and form. Keep in mind that the seedlings may or may not resemble the parent plant in size and shape because most Japanese maples are propagated by grafting rather than seed grown. (In my experience the seedlings tend to be bigger than their parents in addition to the other different characteristics.) Have fun with your project!
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