James,
If you do a search on this forum you will find plenty of good info on overwintering hydrangeas, however, the whole point of planting Endless Summer is that it will bloom on new wood and therefore doesn't "need" winter protection. It's up to you......yg

Hi James. I'm in Missouri, too. The first year I had Endless Summer I protected them by covering them with cardboard and mulch. The second year I did nothing. No pruning, no covering, etc. I might have put a few shredded leaves over the crown, but other than that, they were naked and on their own. I found that they did better after the second year with no protection. In my opinion, the key to success with Endless Summer is to have good soil, amended with lots of compost, and use plenty of mulch. Keep them watered, preferably out of the afternoon sun, and you should be rewarded with happy plants.