The Arbor Day Foundation promotes the Hardiness Zone map developed, under USDA contract, by the American Horticultural Society. The revised Zone map was not accepted by the USDA, due to many questionable assumptions/errors.

Winter temperatures during recent years have, on average, been warmer than than previous years, but we cannot assume that trend will persist forever.
The much discussed "Global Warming" is based on an average temperature increase of only 1°F, not enough of an increase to change plant hardiness Zones by 10 degrees.

I would also recommend caution in Zone upgrades, unless you have a known and consistent microclimate area that will support more tender plants.

I grow a number of plants, hardy only in higher numbered hardiness Zones, but they are protected from lower temperatures during the winter months. It can be satisfying to find plants that will survive colder than recommended hardiness Zones, but I only try ones that are relatively cheap (like me)!
Rb

esh -- I'm in Central GA; and had always thought my zone was 7b...until the last couple of years I was told it was 8a...now, I've no idea. And, it IS confusing...

RB -- I agree. I have had some things work that weren't supposed to, based on microclimates and drainage issues. For instance, last year I went out on a limb and bought those dwarf mugo pines for my rock garden. They are doing really well, but the sedums...in the same garden...died faster than you can say jack rabbit. (Just this moment I am suddenly wondering if it is because the pines had a larger root system and therefore were hardier.) But, as temps rise (ie: the recent predictions of extreme heat by 2012), I'm going to start wondering if all I'll have left to choose from is juniper or juniper or...hmmm...juniper?

I don't have an outside thermometer, but the Weather channel showed a low of 11 degrees here when I checked it at 8:15 am. It is now up to 12 (so it is rising). Tonight could be a few degrees colder, we'll have to see how it goes. The sunshine during the day (yesterday and today) is certainly helping the day time temps, I guess. Still the high today is predicted to be just 30 degrees.

Brrr! This just proves that there are occasionally weather events that bring us to (or very near) the stated lows for our zone, even if it is only every few years.

I hope GaAlan will chime in with his readings and some historical numbers.

3 degrees even this morning at our house according to our thermometer.

Well, the South's famous for its weather fluctuations, more so than many other areas. Nothing new about that. I've personally observed gradually warming winters here and in California over the past 3+ decades, though, so that and the continued sky-is-falling calculations of scientists for the past 60 years or so make me accept that change is occurring. Although people can argue about why, how much globally is pretty well agreed on. Just not here. :)

My temperature was 11°F at daylight and dropped to 10°F after sunrise, now up to 17°F (38°F in the sun!).
It has been 10° colder on this date, since I have lived in this area.
It is interesting to note that while some "scientists" are predicting a global warming disaster, others of the same ilk, are predicting another Ice Age! Guess we will have to hang around for a few thousand years to see how it turns out. :Rb

I read recently that they are predicting more "temperature-turbulent" winters in the near future like the one we have been experiencing this year.

Just a couple of weeks ago, I was thinking, "Well, guess I need to get some garden chores done, since we're going to have all this mild weather for January!" LOL

Oops!

My reading on the subject falls in line with what I am seeing here. The climate is warming and drying, but the main visible problem is variability of weather events. We have always had wild swings in temps here in georgia, but not necessarily the extreme events we have been seeing the last decade or so nationwide.

Seems like we can't get through a summer anymore without record floods, droughts or numbers of tornadoes; winters without enormous variation in precipitation and temperature. It seems it won't be the climate that gets us, it will be the freaky weather systems it engenders.

Another reason to grow natives, I think. Although I am still pretty new to gardening, I find myself more concerned about planting something native (that, of course, does better in full sun, right?) to zones, say, 3 - 7. Is it going to make it through our sultry summers? Am I pushing it?

As a (former) scientist, I sometimes wonder if the climate change debate is as indecipherable to the lay person as the situation in Palestine is to me, someone who is not terribly versed in political science beyond the news headlines.

Not that gardeners (scientists in their own right) and others shouldn't try to read and process the data on their own. It can just get a bit messy. And unlike Newtonian physics (the kind we all learned in high school), climate science is not composed of wonderfully clean, and ultimately satisfying, Laws. There is uncertainty in trying to predict our highly variable natural world. No one knows that better than you gardeners.

I think it is important to recognize that "global warming" might be a bit of a misnomer. Climate change is much more appropriate. Some places will be wetter, some drier, some warmer, some colder, and these climatological changes are already occurring - and will continue to occur - at a rate faster than the adaptive capabilities of many organisms, including possibly some of the gorgeous microbes upon which we gardeners depend. My bottom line is this: I think that simply just because the data is messy and overwhelming, doesn't mean it should be dismissed.

P.S. It's raining again here!! Are wet winters the only thing we can count on?

Hello Ga transplant! Always glad to see new people putting down roots in our red Georgia clay. As a native, I sometimes think we are in sore need of a little hybridizing.

I think you may be pushing it trying to plant things accustomed to such frigid climes as zone three. As with lilacs and tulips, they may grow here for a while, but they will never perform properly. My rule of thumb is to plant for the zones immediately above and below me for expensive things and experiment as best I can with the stuff on sale which is less well tempered to our immediate climate.

The last few weeks I have been trying to remember such a wet winter. I THINK our last one must have been 1990, when it rained for three months straight in the spring. With our Mediterranean climate, winters will usually be damp, but actual feet of rain in two months is unusual. I will defer to those with rain gauges routinely noted and not, as in my case, accidentally run over with the lawnmower.