Pretty much correct. Rhododendrons which enter the winter months after adequate watering or natural rainfall are better able to endure the cold than ones which have been allowed to dry out.

The physiological comparison between rhododendrons and human mountain climbers is, however, not really accurate. In deep cold, rhododendrons cannot replace moisture lost through their leaves from the frozen soil. They curl their leaves tightly to conserve moisture, but once the soil is frozen it doesn't matter if it's dry or saturated - very different from a mountain climber increasing water intake to avoid dehydration. Various methods, including burlap screens and anti-dessicants help rhododendrons avoid leaf dessication by protecting them from sun and wind and decreasing transpiration from the leaf surface.

Don't Rhodies take in water through the leaves? Even if the roots are frozen, can't the shrub take water through the leaves?

I am convinced that living tissue....human or otherwise....suffers more from dehydration in the winter than in the summer. I had a friend that climbed Annapurna and had to turn back near the top because his lungs dried out, with acres and acres of snow around that covered the ground with many feet thick.

Similarly, when my rhodie's leaves droop in the morning after a hard frost, they perk right back up again with a morning watering.

As far as I know, the entire physiological process whereby rhododendrons adapt to winter temperatures is not completely understood. Under certain conditions, rhododendrons probably do absorb atmospheric moisture through their leaves. In temperatures even just below freezing, however, they cannot because the leaf stomatae are closed.

The curling of the leaves into tight tubes may have more than one survival purpose, but the mechanism is entirely thermotropic. That is, it is a response to temperatures at the leaf surface and nothing else. The leaves perk up after a frosty night not directly because of the watering but because either the watering raises the temperature above freezing or because the air temperature is rising naturally as it's warmed by the ascent of the sun.

It is true that dehydration is more of a problem in the winter than in the summer. All species of plants and many species of animals which are adapted to winter cold regions have evolved various mechanisms to shut down or curtail normal transpiration and thereby enhance the chance of survival.

No, rhododendrons do not take in water through the leaves. Through transpiration, they expel small quantities of water vapor through stomata, small pores on the underside of the leaf. The leaves curl to prevent this.

Antitranspirants/antidesiccants such as Wilt-Pruf, are used to block these pores and prevent desiccation. They are only used in the winter since these pores must be open during periods of active growth. They typically are applied twice, once at the beginning of winter and again in mid-winter.

Steve Henning
Reading, PA USA Zone 6

As far as I know, the specialized structures of rhododendron leaves - papillae, scales and indumentum - usually assist in transpiring water in times of excess moisture such as the Asian monsoon seasons. At other times, however, they do the opposite. They conserve moisture by retaining a thin film of water around the leaves during very dry conditions. At these times it seems pretty clear that some absorption of water through the stomata is taking place. If not, foliar feeding with broad spectrum fertilizers or iron chelates, for example, would not be possible.

You are right that some rhododendron absorb water through the leaves.

I know that some redwoods take in water through their foliage. For that reason they grow taller in foggy areas. Apparently they can't force water from the roots that high through normal capillary action, but with water absorbed from the fog this is not a problem.

Apparently chelated iron and some other nutrients have a quicker response when applied as a foliar spray, but lets not forget that most foliar sprays also enter the root zone due to run off.

The trichomes (hairs and scales) on the leaves of rhododendrons are capable of absorbing water. They perform 3 functions:

1) They prevent the loss of water by transpiration (loss through stomata) in times of drought.
2) They preserve the balance of water in the plant by absorbing water and passing it on to the leaf.
3) They promote water loss and hence increase growth by accelerating transpiration and also secrete water when high humidity retards transpiration.
[reference: "The Rhododendron Leaf" by J. M. Cowan]

Since nutrients are carried from the roots by water, the loss of water by leaves brings up more nutrients from the roots. Perhaps that explains why many rhododendrons thrive more in the dry Mediterranean climate of the Pacific Northwest than they do in the humid climate of the East Coast.

Having said this, the most efficient structure to take in water in rhododendrons is the roots. Since during cold weather, many rhododendrons curl their leaves to prevent transpiration, this also inhibits the trichomes on the undersides of the leaves from absorbing very much water. Also, the winter air is quite dry which means that there isn't much water to absorb. In humid but rain free summers, the trichomes are much more efficient in absorbing water.

"Dry Meditteranean climate of the Pacific Northwest"??!! Coastal Washington, Oregon, Northern California and British Columbia (which has the only true temperate rain forest in North America) are ideal rhododendron growing areas precisely because they have high rainfall and a high level of atmospheric moisture combined with year round moderate temperatures. Humidity combined with 90+ degree days and nights such as much of the East experiences limits rhododendron growing because of the high temperatures, not the humidity Among other things, the extreme heat encourages the various root rots and other fungal diseases. Go to a humid but cool Eastern area such as Cape Cod, coastal Maine,along the Great Lakes, the Niagra Valley of Ontario or Nova Scotia and you'll see many rhododendron varieties which can't be grown in hot inland locations flourishing. In fact, nearly all of the world's major rhododendron areas from Scotland to New Zealand are characterized by high humidity and moderate to cool year round temperatures - just the opposite of a Meditteraean climate

One fact most often overlooked is that we have relatively dry summers here west of the Cascade mountains. So for about three months our growing climate is close to a Mediterranean one. Maybe not as hot though.
BiggerDiggler lives east of the Cascades with a much drier,climate. Also much colder and fairly dry in the winter, with temps a lot warmer in the summer.
We get our wet reputation because of our long, wet, winter with many overcast, drizzly days and nights.
Mike

The Pacific Northwest has rather bizarre climates. On Vancouver Island in BC, the area around Henderson Lake on the west coast gets 268 inches of rain each year while Victoria only gets 25 inches, virtually none in the summer.

1) The coast has rain forests. Some areas get between 100 and 200 inches of rain in the mountains. But very few people live in such areas. But the coast has some dry areas that only get 20 to 30 inches of rain.

2) The valleys between the Coast Range and the Cascades have the Mediterranean climate with about 30 to 50 inches of rain in 9 months but virtually no rain many summers. The bulk of the population lives in these valleys. Many rhododendron growers mist young plants in the summer.

3) East of the Cascades is prairie land and desert land. Some areas receive less than 10 inches of rain and virtually none in the summer. There aren't many rhododendron growers, or people in general, in this region.

I worked on a regional forest fire crew in Oregon & Washington for the US Forest Service and we spent most of our time in regions 2 and 3, only once in region 1 near Bandon, Oregon.

Portland and Seattle have a rather unearned reputation for being very rainy. The truth is they have many days of rain during the wet 9 months, but they have fewer inches of rain than the major cities on the East Coast.

The 12 major US cities with the most annual rainfall are all in the East or Texas.
The 12 major US cities with the most days of rain include Portland & Seattle behind Buffalo, Rochester, and Cleveland.
[rainfall data]

Yes, that pretty well sums it up Steve. The diversity in rainfall is amazing. 15 miles west of me the rainfall is approx. 37 inches per year,(Seattle) about half what I get. 15 miles east of me it's twice as much as I get.
I don't have to water my garden in the winter, Fall, or Spring. Drainage is more of an issue then.
BiggerDiggler, do you live in northern or southern Idaho? I assume with winter water issues, you live in the southern part.
My brother lives in the northern half, just east of Spirit Lake, near Coeur d' Alene.
Mike

The coastal areas of the Pacific Northwest have maritime climates, sometimes called Oceanic or Marine West Coast climates. Rainfall is only one factor in what makes this area the prime rhododendron growing region in North America. Atmospheric moisture - which is not the same as rainfall - prevalence of cloud cover and the temperature moderation provided by the proximity of the ocean and the prevailing winds are of equal importance. Certainly all of the PNW states and Canadian territories have significant variation in climate away from the coast and east of the mountains. Characterizing the coastal areas has having a Meditteranean climate solely because of a summer low rainfall period is, however, terribly misleading. Lumping Seattle or Vancouver in with Marseilles or Genoa is just silly.

I live in Post Falls, about 12 miles from Coeur d alene. Weird climate here, supposedly this is zone 6, while at the same time Lewiston, at sea level, is supposedly the same zone 6. This area gets incredible snow fall during the winter (2008, we had three feet of snow one night, with a total of around 200 inches for the year). Huge huge lakes that moderate temperature extremes though. Winter cold is not nearly as cold as Southern Idaho (which has clear skies and abusively-cold nights of the high desert, easily get minus 20 F.), or does it get as hot here in the winter (highs of mid-90's here during the summer, southern idaho can get 115). Lewiston can get 120 and is incredibly muggy, but only gets down to maybe 20 F in the winter. You can grow magnolias there.

Southern Idaho, with its hot long sunny summers, is a rose growing paradise. We lived near Boise when one year there was not a cloud in the sky for some 9 months. Winter segues directly into hot summer. Snow on the ground one day, the next can be in the 90's. Northern Idaho reportedly sucks for roses, although I am experimenting and reevaluating that position this year.

Northern Idaho, where evergreens like Pine are virtually weeds, has incredibly acidic soil. Therefore I think this moderate climate with this soil must be Rhody paradise. Even with new plantings this summer, right in the teeth of high summer temps in the 90's, new rhodies exhibited very little, if any leaf droop or any other signs of obvious distress. Maybe the acidic soil facilitates water uptake? Or does it alleviate some of transplant shock?

OTH, Southern Idaho is living hell for rhodies, very few survive. They boil alive in the hot dry summers, the soil is very very akaline, and the poor rhodies freeze their nuts off in the ungodly winters. People dig those bathtubs and fill the bathtubs with amendments and the poor rhodies die of root rot, even though they are dry as a bone up above.

So as a transplant from southern idaho, where roses grow like magic and rhodies die a sickening quick death, I have reversed my thinking and the rhodies are in heaven, and the rhodies are black spot wonders. Except now I have learned that Roses here do well in a harsh toxic chemical bath and rhodies are best left to their own devises.

My Mother grew some Rhododendrons in Millwood, Wa., just west of you. They were in partial shade on the east side of her house. They seemed to do just fine. I don't remember the varieties.
Mainegrower, I only compared the summers. I spent a good deal of one summer near Malaga, Spain on the Costa del Sol one year, and the climate at that time seemed very much like western Washington. Not silly at all.
Mike

When considering the climate needs for any genus of plants, we have to look at such things as rainfall, temperature and humidity year round over a considerable period of time, not one year or one season.

Suffice to say, the coastal areas of the PNW do not have " a dry Meditteranean climate." The great rhododendron growing areas of the world - for both natural species and introduced hybrids - from the Himalayan foothills to the Smokey Mountains to Vancouver Island to New Zealand are characterized by moderate temperatures, high levels of atmospheric moisture, sun filtered by frequent cloud cover and adequate rainfall.

Pax. Finis.

Victoria, BC, is on the coast of the PNW and describes their climate as a Mediterranean climate. They get about 25 inches of rain, but virtually none in the summer. Mediterranean climate doesn't mean dry, it means a dry summer.

Here is the technical jargon that explains why this is called a Mediterranean climate:

Under the Koeppen climate classification, a cool summer version of the dry-summer subtropical (Csb) designation, typically referred to as "Mediterranean", is assigned to many areas of the Pacific Northwest as far north as southern Vancouver Island and the Gulf Islands, including cities such as Victoria, British Columbia, Seattle, and Portland. These zones are not associated with a typical Mediterranean climate, and would be classified as Temperate Oceanic, except dry-summer patterns typical to the Pacific Northwest meet Koeppen's minimum Cs thresholds. Other climate classification systems, such as Trewartha, place these areas firmly in the Oceanic zone. [Pacific Northwest Climate]

Yeah, the Koppen system has some shortcomings, which apparently have led some to try to revise it:
http://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification#Trewartha_climate_classification_scheme

I think you are both right...it's not a 'dry Mediterranean' climate in the PNW, but it's a cool summer one. (nor is the entire Mediterranean basin quite as warm as is assumed by some Americans...Barcelona's summers are barely warmer than Boston's. Although Rhododendrons don't seem to grow there because the soil is alkaline, there are plenty of huge, lush looking hedges of Pittosporum tobira. I have no doubt if an ironclad were nursed for a few years in the right spot, i.e. acid soil, it would survive. Jean Marie has been documented having roots > 2' deep.)

Even around Seattle, they do, in fact, have to water plants like rhododendrons and kalmias in the summer, particularly if they are not established. But in a place with such cool summers, a little watering goes a long way. Here on the East Coast I've had lucky summers when I've not had to water a single newly planted plant. Even perennials. Of course, this summer wasn't one of them! I had plants that would literally be fully watered one day, and fully limp 2 days later. Hot air is able to hold a lot of moisture, and even when it is moist, it can extract a lot of moisture from the plant.

As for rhodies in a place like Idaho...well...the cold and dryness you can experience is much more severe than most of their native haunts. However, part of SW China and the Himalaya have very little rain in winter. However temperatures tend to be stable compared to ours because the Himalayan anticyclone doesn't move very much, and the mountains and other ranges in Asia block outbreaks of severe cold. (for example it's slightly ridiculous anybody tries to call the highlands of NC a 'subtropical highland' climate, when their record lows are all around -15F if not lower...whereas the real thing like Kumning, China, has a record low of 20F...the Himalayan hill towns are even milder) I'm pretty certain there would be a steady, light wind in winter there, versus our alternation between cyclonic weather (snow storms) and intense periods of drying wind as frontal passage occurs. Compared to what we experience, there are really only 2 major frontal passages per year in the land of the Laligurans...SW Monsoon in June (wet) and NE Monsoon (dry) in October. They might have other minor fronts throughout the year, but with lower pressure gradients that ours.

One interesting mystery I've been meaning to mention, that ties into (sort of) this thread. Last winter I was on the West Coast and observed, at several gardens, rhododendrons growing in low spots that looked like, with a large enough winter storm, they would almost temporarily flood. Again, considering they come from an area that gets rather dry in winter, this would seem like a problem. Certainly, spots like that on most of the East Coast would be a major no-no. For the reason of rot in _summer_ that Mainegrower mentioned. But these plants seemed fine. My theory is that, of course, this wet soil occurs in winter, when the plant isn't actively growing (but of course, still has some physiological need for water) at a time when soil pathogens are not going to be a problem. (unless there were literally water covering the ground all winter...of course that would kill the roots but the 'pathogens' in that case would be merely opportunistic) In the summer, in most of the areas, the plants are at best going to be skating by on reserve soil moisture, at worst going to need watering. I'm sure, for example, even at the Berkeley Botanic Garden (72F max. in summer) the magnificent Himalayan rhododendrons have had to be watered for some years until they were established. But they don't have to worry if the soil is a little moist in winter, even though it's known for being a sticky clay. It just isn't going to harm the plants.
Whereas on the East Coast, we have to site things very carefully for those times when we get ridiculous deluges during the hot season. Luckily this year I only lost a couple rhododendrons to root rot, after incredible rainfall in August & Sept. Most of mine are planted on slight slopes or small mounds, I have no doubt this saved a # of them.

It's probably just a little to dry for a rhody ever to last on its own, but only just barely. The main problem is - compared to warmer, drier places in the PNW like Roseberg, OR - that the rainfall peak is actually during the rather warm autumn. And because of that warmth a lot of the rainfall is going to evaporate/transpire versus the peak rainfall being in the dead of winter (Jan-Feb) as it is for the PNW.