First my sympathies at the loss of your fabulous hydrangea tree. Root rot can be caused by several pathogens, but it is poor drainage that encourages the growth of the bacteria and other problems with the root system. Hydrangeas are susceptible to root rot if you soak the soil too long with a hose especially in the summer when the soil is warm. I suggest you replant in the same spot if you want but to first amend the soil with organic matter such as leaf mold or compost and be sure to loosen up the soil in the planting hole so that it drains well. Do not plant if the soil is soaked with moisture in early spring. Wait until early summer when the soil has dried out. Next be sure to water the first year when the top two inches of soil is dry but do not soak the soil with a hose left nearby or by allowing the runoff from a gutter or downspout to saturate the soil during warm weather. Your goal should be to train those hydrangea roots so that they seek enough moisture out on their own and don't require extra summer water. Err on the side of barely enough water not overwatering. The Hydrangea paniculata or Pee Gee hydrangeas are the best for training into trees and surviving on summer rainfall in the Pacific Northwest. Keep growing, Marianne Binetti