Asked September 21, 2019, 12:53 PM EDT
Ramsey County Minnesota
Thanks for the question.
Hydrangeas will keep getting bigger unless they are pruned. What you experienced with your hydrangeas after your pruning is quite normal. Every spring I prune about a third of my hydrangea stems down to about a foot above the ground and right now they are approaching seven feet. Digging parts of it up would not be wise. You may end up damaging the remaining roots leading to possible loss of the entire plant.
As you consider your pruning procedures, it is important to know if your hydrangea blooms on “old” or “new” wood. Hydrangeas blooming on old wood develop flowers early in the season (June). Hydrangeas blooming on new wood develop flowers during August and September. I would guess that your hydrangea is in this second category.
Perhaps you may want to consider something called rejuvenation pruning for your hydrangea. Take a look at the third paragraph on the second page in the following publication:
Here are some other publications dealing with extensive pruning:
Hydrangeas are vigorous growers and will require annual pruning to keep them to the size you want.