Answer: A forsythia this old will have a huge root system (deep and wide and very heavy) and be very difficult to transplant as a result. You might want to start a new plant using a tip cutting or by layering it, or you could try to dig out a division of it and move just part of it. The best time to try to move all or in part would be in the late fall or very early spring as soon as the soil can be worked. You would trim back the branches to compensate for the roots lost in the digging, replant immediately at the same depth as it grew before, water deeply to settle any air pockets, and mulch with several inches of organic mulch. Cuttings and layering can be done now. I would recommend doing a few cuttings to have as "insurance" in case the transplanting does not work out. Here are directions on how to do all three.
Good luck with your project.
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