Answer: Hydrangeas do best in an evenly moist soil that is also well drained, and a humousy soil is preferable to heavy clay. You might prepare the planting spot by digging a very wide hole and working in lots of organic matter such as compost, peat moss, well rotted leaves, or aged stable manure and bedding.
In general the best time to transplant is very early spring or early fall. If it is one of the macrophylla types your plant should be fairly shallow rooted and hence relatively easy to dig up and move, although it is sure to be extremely heavy. Water it well the day before you move it. Trim it back short, carefully dig it up taking as much of the root ball as possible, replant right away and water it in well. Apply several inches of organic mulch such as shredded bark to help conserve moisture and keep down weeds. Expect it to take a year or two to recover and be careful to keep it well watered for at least a year, just as you would a new plant.
Since I don't know which type of hydrangea you have, I will mention that if it is one of the larger and woody or shrubbier types of hydrangea (such as panicle hydrangea) it may be very difficult to move and should probably be root pruned during the year ahead in preparation for the move.
Q&A Library Searching Tips