In the fall when the plant starts to go dormant and the temperatures are cooler, start digging around the base of the baby clump near the mother plant, starting with the outside of the plant and work your way in.
The roots can be pretty tough so use your body weight if needed to dig through and under the roots. Try to salvage as much of the root ball as you can. The more protected the roots are, the easier it will be to acclimate to its new location.
Dig around the base of the clump in a circle and use a lifting motion when digging under the plant to start separating it from the mother plant.
Use a garden fork to get underneath and help lift the root ball out.
Continue digging and lifting. It took me about 10-15 minutes to get two clumps out but was well worth the effort.
Immediately replant the baby clumps in another area then water it well.
Follow along to learn more about how to divide hydrangeas.