It surprises me how many people still don't understand the idea behind a climate zone map, including those who should know better. We had an article in our local paper today about the new USDA map (which puts most of Ohio in zone 6). They interviewed a manager at our largest (and best) nursery in central Ohio (which was previously placed on the border between zones 5 and 6 and is now all zone 6). He said he was continuing to tell customers that we are in zone 5, because we're still going to have extreme winters or severe storms from time to time. That's not the point of climate zones, which are determined by the average winter low temperature. If zone 6 gardeners see an wintertime low on average of -5F but are faced every decade or so by a drop to -20F, they are still in zone 6, not the border between zones 4 and 5. A severe cold snap might damage or kill susceptible plants (depending on site, drainage and snow cover) but it doesn't make sense to garden with these extremes in mind. Of course you could play it safe and only use plants that are semi-guaranteed to be rock-hardy in your area, but you'll miss out on a lot of great things. If (for example in my area) you get years of enjoyment from a Nandina or crepe myrtle but it eventually succumbs to a hard winter, you're still ahead of the game.

No discussion of the temperature record is complete without debunkning the myth that the temperature record is unreliable or somehow skewed by heat islands. http://skepticalscience.com/surface-temperature-measurements.htm It's valid. The planet is warming. Man's activity is making a significant, measurable contribution to that warming. We (species) are set for "interesting times" Here is a link that might be useful: http://skepticalscience.com/surface-temperature-measurements.htm

I went from 7 to 9A according to the new map(small town, not very populated, mountains so one town is up / colder and one lower elevation and warmer, most maps had been pretty generic). I guess you could say my average winter night temp is in the 20's but I think of USDA 9 as my bay area location and can attest to the fact that a lot of the plants I brought with me cannot handle the winters here. Sunset zones still have me at their 7 which is more useful for me (I have SUCH a shorter growing season here). Our record lows tend to be high single digits to low teens, so I'll probably use that - like others every winter it seems we're hitting a "record low" and I don't want to have to run out and cover plants in a snowstorm.

Thanks Carol, Great post! I really want to study this to see how valid it is to help make decisions where to move (final downsizing). We are a block from the river and 11 miles from the ocean. As a result we get a lot milder weather than folks just 4 miles from here. I spoke to a friend who lives not far from us. We said her bird bath had a layer of ice, ours never froze. We never had a freeze this year, but usually get 1-3 freezes. Our area is full of micro climates. Thanks for sharing this link, Bob