The need to winterize will depend on the type of hydrangea. Those that bloom on new growth - paniculatas, arborescens (Annabelle's) - are very cold hardy and do not require winter protection either to survive or to produce blooms.

Macrophyllas, bigleaf hydrangeas, are not nearly as cold hardy. Most are only root hardy to zone 5, meaning that top growth is likely to be cold damaged if left unprotected. And since the vast majority of macs bloom on old or previous seasons' growth, this means you will miss the bloom season. Even those newer, increased hardiness remontant (reblooming) types like Endless Summer can benefit from winter protection in zones 6 and below to preserve the older growth. Since these plants also bloom on new growth, you won't necessarily miss the bloom cycle entirely but it will start later and won't be all summer long :-)

Same with the other species that bloom on old growth - serratas, quercifolia, aspera, etc. Protection in zones 6 or below is advised.

Thank you! That is very helpful!
What's the best way to protect hydrangeas? I've heard placing oak leaves around them may work?

Hi Anna. I too live in IL. I just posted the following experience with Endless Summer in another post:

"I planted several endless summer last year. I did not do anything special for winterizing, no trimming, no mulching or covering. I live ~ 40 miles north of Chicago. We, as usual, had a very long and harsh winter. And there were times the plants were not covered in snow for extended time period with very cold temperature.
They all survided. We started to see some spring weather and temperature, so I went out today and examine the hydrangeas. New life started to grow out of all the endless summer plants.

A couple of my neighnors also have endless summer for several years. They never do anything for winterizing. And every year the hydrangeas came back nicely.

They seem to be very hardy for our Zone 4/5a, and long cold winter climate."

However, Endless summer very much dislike afternoon sun. if you have mostly morning sun, little afternoon sun, they would be the best. Of the endless summers I planted last year, the one that did the best (long blooming time, easiness to take care, healthiness of the plant, etc)is the one that only receives a couple hours of morning sun. Otherwise it is a constant battle to keep the plant and flowers from wiltering in the hot summer days, you almost have to water them every day.

Good luck and All the best,
Vivian