Your comments in "My soapbox on the USDA hardiness growing zone map/system" make some good points, but there is a lot more to plant hardiness than temperatures. Most plants can withstand an occasional dip below their hardiness range, but they cannot tolerate sustained periods of temperatures lower than they are accustomed to. You are probably aware that some plants do not tolerate heat well, if at all. Moisture is an even bigger factor, as you must know. Many genera of plants that can withstand cold temperatures, can only do so if they are kept dry in winter. I have a friend with heavy clay soil who loses things over winter that come through just fine in my well drained sandy soil. On the other hand, I have difficulty keeping moisture loving plants alive during any time of year. Humidity levels play a big role in the success or failure of many plants. A gardener in zone 5 in an arid region will have quite different experiences from a zone 5 gardener in a region such as ours where the lakes keep the moisture levels in the air on the high end of the scale.
Then to confuse us even more, there are 2 widely used zone hardiness maps. The map published by the USDA, and the one compiled by the Arnold Arboretum at Harvard University. The maps are pretty close in agreement on defining where the boundary lines lie between zones, and they also agree on annual average minimum temperatures for each area. However, they differ in numbering scheme, and therein lies a BIG problem. Areas on the latter map have numbers areas by one less in value than on the USDA map, so it places us in zone 4. I personally find it difficult to stay focused on the idea that a zone 5 plant in some references and catalogs is really a zone 6 plant if the source I am referencing uses the Arnold Arboretum map. Wyman's Encyclopedia of Gardening is one of my favorite references, and it uses the Arnold Arboretum map. Often puzzled by why a plant failed for me when the text says hardy to zone 5, the light finally goes on and I remind myself that I am not in zone 5 according to that particular reference. Even more difficult are the plant tags at the nurseries which give absolutely no clue as to which map the grower follows. I am certain this is why I cannot successfully grow some of my purchases.
I have tried to discipline myself to look for the hardiness map first when a new catalog arrives, but I am a slow learner! I often have my wish list ready to whittle down because I can't possibly afford everything that jumps off the page yelling, "BUY ME!", when I stumble onto the page with the hardiness map. That is when I discover that half the selections on my list need to get crossed off anyway because they aren't reliably hardy here. Maybe I should just train myself to only consider plants listed as zone 4, then it wouldn't matter which map the book was using, I could rely on hardiness in my landscape. But then that would introduce a new problem ... I might pass up some zone 5 plants that were actually hardy here, rejecting them because I was only considering zone 4 plants. Oh, yes, and then there is the numbering used by Sunset, but I'm not going to go there because I have never seen that map, I just know that in one of my bulb catalogs which references both that system, and the USDA map, the numbers are really, really different!
And then there are micro climates to be considered. I know that my landscape benefits from a warm pocket because I don't have frost as late in the spring nor as early in the fall as the yards around me do. I attribute this to several things including the canopy of trees over the back yard, and the wooden fence surrounding it. Both help to keep the heat trapped inside the perimeter of my yard. I would assume that this could mean that it is always a little warmer, at least by a few degrees, in my landscape than in my neighbors'. That doesn't necessarily mean that I am warmer than locally recorded temperatures. It could mean that they are colder. Or maybe I am drier, so there wouldn't be any moisture to freeze and form frost. Whatever the reason, I definitely have the benefit of a different micro climate. I have successfully grown some zone 6 plants for years, but I have also lost plenty of plants I took a risk on trying.
As to the problem of the same plant being rated for different zones by different sources, I think some sources just don't want to risk folks being mad at them, or trying to sue because they trusted their information. Some plants haven't been tested outside their native habitat. When I accepted the position as Chairperson of the Hardy Cactus a Succulent Garden on campus at MSU, I read every piece of information I could track down on the subject. I had difficulty finding any reference to Opuntia humifusa (or any other species of Opuntia for that matter) as being hardy any further north than zone 7. I knew better, because I had been growing them in my landscape for years. Eventually, I came across some magazine articles that mentioned the fact that they were native almost as far north as the Arctic Circle, yet there are still many mail order catalogs that do not list them as hardy here.
About your rhododendrons ... what were the growing conditions? I assume you amended the soil as necessary to accommodate their love of acid soil, but how much sun were they getting? Too much, maybe? I grew up in W.Va., and rhododendron is the state flower. In its natural habitat, it grew in the woods in dense shade surrounded by lots of trees including tall evergreens. It didn't venture out of the woods except on the north side, and even then it tended to stay within the boundary of the woods. Every gardener that has ever complained to me about their rhododendron admitted to planting it in a fairly sunny area. Those that I convinced to move their plants into the shade were eventually happy that they did. Maybe there are newer cultivars that can tolerate some sun, but I'll bet the key word is tolerate. Why antagonize your plant? Give it a space it can comfortably settle into, and reap the rewards of a plant that dazzles you with evergreen foliage, and wonderful flowers every spring. Never mind what might happen if a plant survived to -10F in z7 in Tennessee, nurseries already offer plants locally that are not hardy to zone 5 or even zone 6. They will sell us anything we will spend money on! I almost bought a plant recently at a local nursery until I thought to look at the tag, and saw that it was listed as zone 7. I wasn't willing to attempt that one. Guess that makes me bad because I ignored points 4 and 5 of the Horticultural Creed ..
4. If it will not grow in my zone or is prohibitively expensive, I want it most of all.
5. I am perfectly willing to forego any necessities of life, such as food for my children, in order to have it.
The rating surprised me because it was a genus that I already grow, and have never experienced problems with hardiness. But I am aware that not all cultivars of a particular genus share the same degree of hardiness.
You would probably enjoy reading the book, Passionate Gardening by Lauren Springer & Rob Proctor. In chapter one, Common Ground, Rob's first article, titled, "The Killing Fields", talks about some of the difficulties with zone designations. He states, "...I'm always pushing the limits. I'm in zone denial, refusing to recognize the limitations imposed by the almighty USDA Zone Map."
Zone ratings, though not perfect, do have their usefulness. However, be certain to take other growing requirements into account when deciding whether or not to purchase a plant ... humidity levels, moisture requirements, wind tolerance, light levels, etc. Oh, yes, and don't forget concerns of the 4 legged critter kind. If you know you cannot provide the needed cultural requirements, then maybe your money would be better spent on a different species. I know what you mean about wanting to grow Yucca (Hesperoyucca) whipplei. I too, have a wish list that includes plants with questionable hardiness.
Enough of my soapbox on your soapbox.
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Alicia Ellis is Chairperson for Genesee County's Hardy Cactus and Succulent Garden on the campus of Michigan State University, and Past President, Michigan Master Gardener Association, Genesee County Chapter, 1998-2000. I met Alicia Ellis at the 2001 sale of my local cactus club, the Michigan Cactus and Succulent Society. The above article is from an e-mail to me dated Wednesday, September 4, 2002.