I would guess that the wilt comes fromt hem being new plantings with under developed root systems. I have mine in afternoon sun and in between waterings they may droop but by the time the sun leaves or I give them a drink they perk back up. The Annabelle which has been in the ground longer than the Nikko or the ES doesn't wilt in the sun. Nikko tends to wilt quicker than ES in my bed.

Based on my observations, it's doesn't matter what is the name of the hydrangea be it ES or Nikko or any other fancy names, mature plants, if watered well in their young years could establish very extensive root system which could support them later if unfavorable conditions occurs.
Of course, full sun in z4/5 and full sun in z7/8 are two different animals and it's my notion that in colder part of the country, up to z6 you could get good results by planting hydrangeas in full sun.
I personally know one very old hydrangea Preziosa planted in a full blazing sun in my z6b on a side of the road where 95-100F in summer would be a rather norm and it still grow and prosper there.
Most interesting fact about this particular plant, it blooms reliably every year despite being open to all winds and colds in a winter and having zero protection from humans or surrounding plants as you could see.

Thank you ego45 and Angelina. My best guess was it has alot to do with an immature root system and the young small roots not being able to handle the direct sun and heat the leaves receive. If it was the tenderness of the leaves the older Nikko's would have fried-up years ago. And I know they have seen many 93+ degree days.

My plan is simply to give the young ones extra water therapy so their roots develop.

The cooler climate is a big help too. Forecast for the next 7 days is a high in the low 70's with a mix of sun, clouds and rain. But I fully expect at least 10 more 88+ degree days coming. +/-

This year I will probably cut the blooms a bit early so the plants can focus more on growing and less on blooming. Cheers, JK

Interesting observations from all.

I live on the coast of southern Me.. For me, Nikko Blue has been a complete "bust". In 5 years, I have yet to coax a single blossom. I protected it last year, as per Hay's guidelines, and have yet to note a single bud.

My Hydrangeas (2 Annabelles, 2 PeeGees, 2 Oak Leafs, 2 Toyko Delights, a Nikko Blue, and a varigated blue one) never wilt. But, they are in partial shade and the character of the soil here is deep and rich. Our lot is "low" and even in times of extreme heat and drought the lawn remains green and vibrant.

I wonder if the "wilt" is like that of the perennial, Ligularia? The root system simply cannot keep up with the rapid respiration that occurs in direct sunlight and higher than normal heat? The fact that your young plants seems to recover so quickly would indicate that.

Very interesting and I agree to an extent...I think Chelone hit on a good point...soil type matters. My older hydrangeas in way too much Florida sun don't burn too much as long as I water them like crazy. A newly planted one has no chance in even less than that kind of sun down here. My sandy soil will not retain enough moisture to protect even mature plants from burning without daily (and sometime twice daily) watering. My newly transplanted (out of the ground and into large containers) hydrangeas handle the heat just fine because they are in great moisture retaining soil that doesn't dry out nearly as fast as my ground soil. Just the opposite of the norm....Go figure.....yg

Thank you Chelone and Yellowgirl. I am situated fairly high in the mountains, 2000 ft on the side of a ridge...so not low. Soil is quite rich with alot of loam, manure, compost and peat moss and 3-4 inches of wood mulch. The young ones have only been in the ground 2-4 weeks and it is their first experience with full sun.

Yellowgirl: That summer sun in FL must be blinding. I would wilt in that weather. Now is it the leaves that need the water or is it the roots??? I am thinking it is the roots?

The other day I was thinking that burying a drip-line hose underground might be an easy low labor remedy. Very slow controlled release of water when it is hot. Easy to install too.

It's definitely the roots...

The dripline would be a perfect low maintenance arrangement for you.

Must be nice to garden without melting!! How hot is it?...At 7:30 this morning, I was watering the plants and I ended up as wet as the hydrangeas, and no I didn't get sprayed by the hose!!....yg

At 7:30 it was 64 up here. 72 full sun at 11:53 and no wilt at all.

That drip-line hose might work very well for you too then. No leaf spray and no evaporation. All you need is a garden spade to lift the soil and insert the hose. And an extra pair of hands to stuff the hose. Then no more running around watering at 7:30 AM.

Nice to hear you you think its the roots which need the water, Cheers, JK

64...oh, you're killing me!!

I actually have an irrigation/sprinkler system (can't even grow decent grass down here without one) that goes off around 4 am. It works as a decent primer but then I just give the ones that get more sun a little extra before I go to work. (I also think there is something sort of therapeutic about hand watering but that's another topic) While it is the roots that need the water, overhead watering IMO has been somewhat unfairly maligned. Mine receive nothing but, even in sun, and I don't think I have anymore or less leaf problems than the next person. Maybe the 'type' of water makes a difference but so far I have seen no ill effects from overhead watering. That is, after all, the way mother nature does it! Besides, a deep drip would just send/waste the water through the sand and right past the roots. In my case, I have found frequency to be more important than depth. Such are the perils of growing hydrangeas on what might otherwise be a beach.....yg

I planted my ES on the South side of my house, which gets full sun from about 4-7.. Originally I planted it behind a weeping cherry tree which filtered out most of the sunlight after about 4. It did pretty well as long as I watered it daily. Sadly enough, the weeping Cherry didnt make it this year and we ripped it out. I am debating whether or not to move the ES which seems to be doing alright, a little wilt on really sunny days. My ASB that I have planted there for about 4 years handles it a little better. Perhaps more established roots. Oddly enough, I was cleaning out my garage this past week and found the tag for my ES, it says "PLANT IN FULL SUN" so we will see. The older hydrangeas in my yard are taking the sun/heat pretty well this year, way better than they did 4 years ago thats forsure.

Ego- Beautiful Hydrangea. My Preziosa is my "old Faithful" too.

Yes. After a couple weeks of observations I am inclined to thinking that young and immature root systems have very much to do with the wilt I have seen in warmer temps. In addition I think that both Nikko and ES are very much more susceptible to wilting when they are carrying blooms, which I think may overburden the root resources of the plant. Too much resources are devoted to flowers and blooms and thereby seeds which are what most plants strive for...aka reproduction. So the wilt on the leaves may only be a symptom---of a not too robust root system which is in development.

I recently cut the expired flowers on some of the young Nikko and ES and suddenly the plants do not wilt. The leaves remain robust and I think the plants resources are now devoting full energy to root and cane growth. Nothing at the moment is channeled into flowering. My best guess.

ie: ES carrying 12 blooms wilt much slower then a plant carrying 20 blooms. And a plant with no blooms wilts even slower.

Thats it. My humble observations. I hope this info is of some help to someone somewhere trying to better understand hydrangeas, over& out, JK

ps: I also find that one should not get too very neurotic about the wilt. Seems here that as soon as the shade comes--the wilt is gone. Here little reason to get out the emergency hose and water them down immediately. In zone 9 that may be a very differnt story.

Hi Jack,

You are correct. Wilt is not in and of itself a reason to grab the hose as it is not necessarily a sign of dry soil but heat stress to the leaves/stems/blooms. Watering is only necessary when the wilting fails to correct itself afer the heat source is gone.....yg