I don't have the facts but I have a opinion, I think you can add the next zone to what you have now, in most cases.

I have been watching the numbers for quite a few years now, and I think you are more or less correct, maternut.

The Arborday hardiness map is much more current than the USDA map. Some areas may be off a little, but I'd say it's fairly accurate.

Myrtleoak, you should write Tony Avent and ask him if he has any news on the release of a new map. He was supposedly on a committee or something like that. I bet if you stroked his ego a little, he'd write you a book on the subject. LOL

A zone up? Are you serious? It must be me again. Everybody is a zone up, and I have been here for 10 years trying to figure out whether I could be a zone down.

For your own garden, look at the temperatures in 1985 for just how cold a winter can be. (Also look at 1898 which was so cold down south that the north end of Mobile Bay froze solid.)
If you choose to dismiss those years as too cold to count, within the past two decades there were temps well below zero at least twice in my yard, yet 6b some years is too cold to describe my conditions and at least twice those below freezing Fahrenheit temps were real garden breakers and made 6b look good, wistfully.

What temps would you choose to average? I can see why the USDA has a hesitancy.

I think their hesitancy (if one is to call it that) is based on an entirely different set of issues. Conditions have clearly changed, and the old data simply isn't correct now. Neither the "official" excuses or the popular conspiracy theories rely on events like in 1985.

I think it's completely valid to plan on future winters being colder or hotter than the current average, but I see no point in assuming the weather will return to the way it was a couple of decades ago unless some major climate changes occur.

When I choose plants these days, I assume I am near the middle of zone 7 (although I'm in zone 6b, according to the USDA map). When I choose trees (a more long-term deal), I usually look for ones that will be OK in low 7a to at least 8a. Unless something major happens, there is no reason not to assume that our climate couldn't continue to get a little warmer than it is now. I also assume future years may bring us droughtier conditions. Many of the respected climate gurus predict conditions in TN to be drier in future years.

ann, I don't think you realize what exactly constitutes a zone. It is an average; even with several very cold years averaged, the numbers are still way up from what they were. 6b means that the average minimum temperature for your location is below zero. It does not mean that the coldest single temperature for a time period was below zero. Plant hardiness is not a stone wall; a zone 7 plant will not necessarily immediately die once it reaches zero. If we went by your reasoning, then sabal palmetto is not hardy to coastal carolina because it reached around 10 degrees there in 1985. Obviously it is, and is in fact native. It once reached below zero on the gulf coast in the 19th century. Does this mean that the gulf coast is zone 6? Hardly. Trunked palmettos are now being grown in Atlanta. If you are doubtful, go there and visit. Come visit the UT gardens and see what is growing on a long-term basis. Many plants ranked zone 8 are doing fine in Knoxville. Also, are you in Knoxville? Temperatures drop very quickly as you head north of the city. Areas of the upper plateau can have lows 5-10 degrees colder than Knoxville.