Last year was my first year with an Endless Summer Hydrangea. I swear to you that I read, on a special Endless Summer site, that when you get a new plant, you should be hard-hearted and cut off all the buds so that the plant can produce bigger flowers. I did that and all I had was a lovely green Hydrangea and a husband who will never let me live it down!
Over the winter, I covered the plant completely with crushed maple leaves and weeds but no evergreen clippings. And here in southern NH, Zone 5, we had a very snowy winter.
In the spring, I took off all the crushed leaves but not until after our last frost, even though the plant was up and looking very healthy and showed signs of blooming.
I waited a few weeks and then I was able to see which old wood had blossoms or green leaves on it. I cut the old wood above that point. Some of the old wood had no green leaves but I was amazed at how many old shoots had leaves and beginning blossoms on it.
I covered the small garden in which I have just the hydrangea, with mulch but I made sure there was an area about 3" out from the main stems that had no mulch on it.
I mixed a couple of handfuls of Aluminun sulfate into the soil around the plant because the numerous flowers that were coming out weren't as purple as I would like. I don't know if it was that or time but they're turning a pretty purple but still not as dark as they were supposed to be.
I've been feeding it Miracle Gro, on and off, although early on, I gave it some Super Bloom on it, early on, which is 12-59-6. A guy told me that everything blooms better with it, again, my husband doesn't agree.
At this point, to my delight, I have about thirty blossoms on this plant and I think I shall cut the small ones off of it today so it use its energy to form more large blossoms. I might be wrong so wish me luck.
Oh, and I try to keep it evenly moist although we've had a couple of periods of nothing but rain. And last year, I mixed coffee grounds with the soil it's in because it was supposed to help..
Hope this helps you! They are such a pretty plant and up until now, it was hard to grow any hydrangea in southern NH. Oh, and I forgot that early on, we mixed quite a bit of sand with the soil we were planting the Hydrangea in because it had too much clay.
Good luck and enjoy your flowers and remember, don't cut off any dead wood until next year, when you're sure it's really dead or it's actually got green leaves on it.