Many of the tallest growing rhodies tend to be species and not all that suitable for sun.

For reds, check out 'Nova Zembla' or 'The Honourable Jean Marie de Montagu'. These are considered very sun tolerant (exactly how much depends on your location) and very cold hardy as well. Both will grow to 6' or more. For a white flowered rhody, check out 'Loder's White'. Also very sun tolerant and cold hardy. And mildy fragrant as well. Can hit 6' (or more) under the right conditions.

In my climate these will grow in virtually full sun. But some late day shade is never a bad idea.

I am in 7b. Thank you both for responding.

Click on "Useful Lists" at the Greer Gardens website. You will find a list of Sun Tolerant Rhodendrons, all hybrids, there. Given the variety names, expected height at ten years can be found by cross-referencing to other sources such as hirsutum.info The Whitney Gardens online catalog also has a wealth of useful information regarding height and exposure.

Zone information tells nothing except lowest average winter temperature for a period of years. Many other factors are involved in selecting rhododendrons likely to be successful in sunny locations. Sun intensity can vary significantly from geographic area to geographic area even though all might be in Z7b. Local nurseries, local chapters of the American Rhododendron Society can help.

Even those that grow okay in the open here in the dullest part of the Lower 48 can have the flowers melt if hot spring days occur. Also none of them like hot root zones.

And the interesting thing is dullest part of the lower 48 are still quite sunny (in summer at least) compared to other maritime climates. Seattle has quite a bit more sun hours than London; in turn London is quite sunny compared to some places in the British Isles closer to the Irish Sea. I saw a documentary on PBS about a Scottish castle featured in Downton Abbey and noticed all the rhododendron hybrids, even fairly large-leaved fortunei-looking ones, seem to be out in full blazing sun...which is never very blazing in western Scotland.
So...point being the question is impossible to answer with knowing much more about the OP's location than "7b". 7b in Norway? 7b in Tennessee? 7b in New Mexico?

Outside the PNW, you will be hard pressed to find a tall, truely red rhododendron that is very sun tolerant. Jean Marie seems more lumpy than tall, and will take a long time to get over 6'. I do have one in 2/3 sun but I think that's the limit. White is a little easier; in the mild parts of the northeastern US perhaps a hybrid like 'County of York' could tolerate full sun in a really favorable, wind-sheltered spot.

Yes, 'Jean Marie' is broad-growing and might take decades to grow 6' tall on many sites. We had one here for that long and the back of it may have just crept up about that high before I had the area it was in cleared out a few years ago. If any part of the shrub did in fact ever get fully 6' tall.

And a normal Seattle summer is a 100 year drought in Britain - yesterday it was in the 70s here, one degree F. short of the record. When I was in Surrey I was told the 70s is a heat wave there, although, of course, as elsewhere climate change may be bringing hotter and drier conditions for at least part of the year to England.

maybe i missed it.. but one of the keys to success .. will be water management ...

if i recall .. rhodies and the related azalea.. have extremely shallow roots ... and are rather heavy water users .. especially when in bloom .... and in sun ...

the issue is significantly increased ... and why.. perhaps.. flowers fade/wither faster in more sun ... the plant simply cant pump water fast enough.. to protect the flower ...

so the first and foremost issue.. might be the soil ... and its water capacity.. then latitude .. and many other variables ...

and be leery ... or aware i should say.. of where your info comes from ... there is a highway sign in mid MI ... that notes you are at 45 degrees north ... half way to the north pole... britian is at about 60 degrees.. or nearly 2/3 to the pole.. and scotland.. has to be way beyond that ...

my point being... the intensity of the sun really matters... as to how much sun a plant can take ...

then comparing the PNW and GBrit ... they only have like 5 sunny days per year.. lol .. they dont even know what full sun is ... but i digress ... [nor do they know what a 100 degree day is] ....

anyway. there are a multitude of variables that are IMPERATIVE ... rather than just getting a list of ALLEGED sun tolerant plants ... the plant just doesnt matter.. if the soil wont hold the water ..etc ..

ken

Ken, you should visit sometime. There are things about our climate that could surprise you.

http://www.komonews.com/news/local/51988007.html

That's an interesting article morz. Seattle having a night time low of 71F is a bit mind boggling, it's almost the high. I happened to be visiting the PNW during one of the worse heat waves you had in the early 1990s...maybe 91? I can't remember exactly when my family took that trip. It was no more than a couple years after the Loma Prieta earthquake because there was still evidence of it in San Francisco. Anyhow, although Port Angeles (where we spent the warmest 24h before taking the ferry to Victoria) was in the upper 80s, which they said never happened, we all agreed the humidity was not as bad as eastern US humidity. How could it be, with all the cold, deep water around? So I bet when Seattle had that 71F low the dewpoint was still several degrees lower. Dewpoints at or above 70F is where the discomfort truly becomes surreal.
Anyhow, yeah the PNW is a rather different kind of maritime climate than the UK. Much sunnier in the dry season, alas, also wetter in the wet season. Which might explain why people have a little more luck establishing plants like Phoenix canariensis, at least temporarily until the next Siberian outbreak, in places like London than in even the urban heat island of Seattle. London's DJF precip total is 6 inches, Seattle's is 14.5! Main advantage of the PNW compared to the UK's is the relative lack of late spring freezes over a broader area, instead of just the favorable maritime areas like Cornwall. Hence the reason we can buy Oregon grown hazelnuts and not West Midlands grown ones!* Probably related to the changeability of Atlantic conditions and jet stream movements versus Pacific, which is strongly influenced by the monotonicity of the Asian monsoon/Siberian high. (and all this from a Seattle gardener of many years who extensively studied the climate, a certain Robert Wagner...not my deductions but I agree with them) On UK gardening forums I often spot posts of some spring flower being snatched by a late freeze, at least in frost prone parts of the garden.

* (among other reasons, of course. Fresh UK grown agricultural products
are rare in the US, period. I noticed when I was shopping in the UK that
most of the even the temperate produce came from the European
mainland. I think a lot of UK agricultural land just grows grains, not
fruits and veggies. Spanish clementines? All over the US in winter. But I welcome a correction if I'm wrong. At any rate, my gosh, some of the pit fruits grown in France were absurdly tasty and reasonably priced to boot, at least when I spent a summer in the UK in the mid 90s...also noticed on later visits. OTOH packaged foods like orange juice, of course, are much more expensive in Europe, period. No 'Juice Trains' crossing the continent LOL, as in the US.)

back to the matter at hand...I think we are all agreeing if you're not in the PNW (or southern Appalachian mountains) version of zone 7b, and if it's a 100% full sun area...you are better off finding another broad leaved evergreen. Again, unless you're in some special microclimate like...the middle of Cape May City NJ, away from the salt sprays but still protected by water on 3 sides from both cold and heat - to some degree. Even then it's going to take an "ironclad" rhododendron.

David, I was in Seattle one of those 100F+ days a few years ago. After my appointment, I'd wanted to go to Molbaks Nursery, hadn't been there in some time and I wanted to browse their extensive selection of decorative outdoor planters. Bad idea, something about that flat roof, expansive indoor and gift area was heating up like a pizza oven. Staff was drenched in sweat, mopping at themselves with towels and handing out complimentary bottled water and cold drinks, trying to entice shoppers to stay and spend. It wasn't working, most everyone was having to escape back outside where it was only 102, out of the sickening (literally) heat inside ;)

About the humidity, here on the coast we will get our both coldest winter and hottest summer days with a drying E wind, no humidity. As soon as the direction changes to more typical onshore flow again, temperatures moderate immediately back to our normal. A summer warm day of 85F can drop 20 degrees in a couple of hours time when the wind changes direction.