The rocketing popularity of 'Endless Summer' results from the fact that, unlike most hydrangeas, this one has the ability to bloom as far north as Minnesota (maybe much further). In the south, where hydrangeas rarely bloom more than once per summer, 'Endless Summer' will often produce a full set of blooms twice each summer.
The Endless Summer Hydrangea is slow to break dormancy, and will not start to grow until the ground has sufficiently warmed up. This could be as late as early to mid-June. Be patient, you may only get growth from the base of the plant coming up from the soil.
Once you see growth, you can cut the dead branches back to the new green growth. This is the time to apply fertilizer and Aluminum Sulphate for blue flowers.
You should see some signs of blooms by the middle of July.
Stop fertilizing in mid-August so the shrubs can begin going dormant.
Keep the soil moist until the ground freezes.
Cut the shrubs back to around 8-12". You will need to cover the plant in order to winterize it. This can be accomplished with leaves, straw or other similar material.
Do not cover the plant until it is full dormant. This is usually around the end of November, around the same time that you would normally cover perennials in your garden. An easy trick is to fill a plastic garbage bag with leaves from your yard, and then gently set it on top of the plant. When the ground thaws in the spring, simply remove the bag and there is no mess to clean up. Regardless of how much you mulch, plan on taking the mulch off the plant when you uncover your perennials after the ground thaws.