Answer: Summer is a really risky time to try to transplant your juniper. The loss of roots on such a large plant is very stressful. Combine this with the heat of summer and your chances of success are very low. November would be a better time, as it would allow the tree some time to settle in and begin root growth before the onset of warm weather.
When you dig the tree, get as much of the roots as possible. The more the better. Experts debate the advantage of cutting back the top, but I think reducing it by about 1/3 is a good idea. Dig the new hole only as deep as the plant's root system, wider is okay.
I wouldn't amend the soil with anything. Your native soil is fine. Don't put fertilizer in the planting hole. The plant will be trying to establish a new root system, and additional nutrients are not neeeded until new roots are established
Good luck with your juniper!
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