I Have. Blue Hydrangea With Branches Close To The Ground That Have Sprouted Roots.

Question

When should I separate from the bush and plant in the ground ?

Answer 1

Since you are in zone 4, you can either wait till spring to cut the branch off the main plant or you can cut it from the mother plant now and overwinter it indoors. Cut it but don't dig it up for a couple of weeks. Let it continue to grow roots on its own before it is uprooted. Then place it in a container with potting soil. Overwinter the container in the garage or somewhere that stays above freezing till spring. It will only need water about once or twice a month indoors. Too much water will cause it to rot. In spring, after the danger of frost is past, you can plant it outside.