My several year old Endless Summer hydrangea survived -14 winter of 2015, although the stems all died back, some completely, some about halfway. (I am supposedly 6a, but I think we have 5b winters about half the time). It is planted on the north east corner of the garage, next to the foundation. I do add chopped leaves to the bed every fall, but not on top of the plant. I guess it gets some protection from that.

We have plenty of hydrangeas in Maine. Some years they die back, but come up from the roots in spring.

For azaleas, yes. The American Rhododendron Society has a really nice search feature-

https://www.rhododendron.org/alphalist4.asp?Color=&Size=&BTime=&Hardy=-10&Leaf=E

Endless Summer isn't the hardiest cultivar of H. macrophylla. Anything below-5Fish will have it growing back from the snowline (or basal buds if you don't have snow).

Actually, ES The Original is quite cold hardy - it is root hardy to zone 4. But as with any variety of macrophylla, winter protection is suggested for any zone below 7 to protect latent flower buds on the prior seasons growth from cold damage. Winter protection will also help to encourage more vigorous and early current growth, as allowing the plant to die back to the roots (unprotected in winter) makes emergence of this current growth quite late into the season and flowering on this new growth is also adversely impacted.

Thank you for your assessment on the winter hardiness of Endless Summer from zone 8/9. Having grown it and other H. macrophylla in zone 5/6 for many years, I assure you I know what I'm talking about.

Whoa - just a little snarky there.......

And I am guessing then that you know more than the original breeder/developer. From Bailey Nursery (located in MN) regarding the development of this exceptionally cold hardy mac: "This new variety was discovered in the mid-1980s in the yard of a 5th Grade teacher outside of St. Paul, Minnesota. Dennis Bostrom, a teacher at the aptly named Gordon Bailey Sr. Elementary School, had one hydrangea that showed the unique trait of re-blooming and surviving the cold Zone 4 winters, unlike any other hydrangea he had seen before. Vern Black, Foreman at Bailey Nurseries, lived down the street from Bostrom and had his eye on the hydrangea.

Working with Bostrom, Bailey Nurseries brought the hydrangea into their facilities for testing. The hydrangea was propagated and tested for over 10 years in the Zone 4 climate of Minnesota to test hardiness and the re-blooming capability in such a northern region."

And even a previous responder to this thread noted that ES survived -14F but as a die back shrub. There are also any number of articles online about the cold hardiness of this hydrangea - like this one - but most of them DO caution to provide winter protection to ensure the survival of any stem or basal buds.

No need to be that defensive there.. But all in all, I guess I learned that it's about the buds and flowers that we most worry about and not the plant itself. The plant itself is pretty hardy but the buds and stems need protection. Right?

That's exactly what I said! Endless Summer is not particularly stem hardy for even a H. macrophylla.

When you started your post with "Actually", you are directly questioning the credibility of the information provided. I simply responded by querying your credibility on this particular subject.

Actually, you can start a sentence with whatever phrasing you want.

If you were to ask me if something was hardy to a certain temp, and specify H. macrophylla, i'd ask about snow cover.

Twenty five below with goid snow cover? Many big leaf hydrangea cultivars will survive and bloom well following those years. Five below and little or no snow cover? Many if the same cultivars will die back completely, and a few might just get killed.

Temps can be misleading when looking at what can or won't survive. Mulch, water, location, how much wind a plant might get can be the difference between doing well, barely survivg, or worse, and all points in between.

We fixate on temps. It's the easiest to define. It's not that simple.

In the mid atlantic, i'd not plant big leaf hydrangea that was not a remontant cultivar...such as endless summer. Unless you are far enough south that cold temps bottom out around 10 above.

Well gardengal you were right on the hardiness of the hydrangea. Today I took a close look on my potted nikko blue hydrangea that was in a very exposed spot for the whole winter along with unusually cold temps this year(0 degree farenheight happened a month ago one night) and I see green buds on its stems and new growth starting to emerge from the base! It had no mulch and half of the shrub is gone due to deer damage. Plus in a container plants usually go 2 zones below than compared to being in the ground so it must've went through like -20 degrees. So yes, gardengal now I know for sure hydrangea macrophyllas are very root hardy

Your estimate that your potted plant endured -20° because it was in a pot when the air temperature dipped to 0° for one night is.. confusing, at best.

Zone hardiness ratings for plants are based on the assumption that the plant is planted in the ground, period. By definition, it does not apply to container plants, and the rule of thumb "choose plants for containers that are rated for 2 zones colder than your area" is, as far as I have been able to find, totally meaningless.

And for good reason. Roots of most plants are significantly less cold hardy than the above ground portions, with root damage or death happening at temperatures as warm as 20°. This means that a large number of the hardiness zones in the US are too cold for the roots of most plants - if they get exposed to air temperatures in the winter. Planted in the ground, they usually do not. Mulch, leaf litter, snow cover, and moisture content of the soil (water insulates better than air) will all help keep that cold air from getting to and damaging the roots. Not to mention whatever heat may be retained by the ground itself. But an exposed container? You're asking for trouble whenever the air temperatures drop below 20° for an extended amount of time.

Glad your hydrangea survived it.

I've not added any protection to my ES (the one that I mentioned above) and have paid the price of no blooms for the past 2 years -- according to the NWS we never dropped below zeroF, although I seem to think that we did last year at least 2 days. So I assume that there will not be blooms this year, although we've had more snow but not necessarily when we had the coldest temps.

Next year I will protect it.