IMO, it's much too optimistic to be pushing to change zones based on just a few milder winters. Yes, maybe it is global warming, but maybe it's also just a natural warming trend that has occured, and then reversed, as it has throughout the history of the earth. People seem to forget that there have been several ice ages, but then the periods in between were a drastic change in climate. You may be sorry, IMO, if you start getting too adventurous with pushing zones. All it takes is one normal or below normal winter, which does occur, even during climate changes, to wipe out most of what you've planted. It was only a few years ago that we had severe, severe cold, with little snow cover, in between these relatively mild winters. I'm in Z4/5, and even with the incredibly mild winter up until almost February, when it finally hit, it lasted, and this year my garden is way behind other years, and the spring peepers and spotted salamanders didn't appear until weeks after they have in other years.

Having said all of that, there are and have always been ways in which a dedicated gardener can push the zone limits on many plants. There are even books written about it. Siting and protection are the major factors. I have some Z6 plants that have done well for me for years. I also have tried some plants which all the tags and the books claim are Z5 or below, and have had no luck, some with 3 or 4 attempts. Bottom line: It's nature, and it doesn't follow the rules which we create.

The USDA hardiness maps, in my opinion, do not really work well on a detailed level. Sure, they do accurately describe general patterns of hardiness, but to claim that some parts of Massachusetts and some parts of Mississippi share the same zone for vegetation (zone 7) is ridiculous and is simply a reflection of the way the zone maps are created. These maps focus only on the lowest temperature typically reached in winter. Zone 7 means temperatures generally don't fall below zero. And yes, for Boston and the coastal areas of Massachusetts, that is the case during most winters. Similarly, zone 7 areas in the upper south also experience similar low temperatures. BUT....and this is a big BUT....the duration and frequency of very cold and windy airmasses in winter is MUCH higher in our area than in the upper south. So while southern areas will have one or two mornings during winter with temperatures in the 0 to +10 range during a cold wave (usually with clear skies and calm winds) they warm up rapidily and that is the end of it. We, on the other hand, have repeated incursions of very cold air, often on strong winds, during the winter. And that makes a huge difference, particularly in late winter when the sun angle is high. Just imagine this: Every winter morning could drop to +1 degree F and that would technically be a zone 7 winter! BUt few if any zone 7 plants could withstand that. And this past winter was also, technically, a zone 7 winter because the temperature never dropped below zero in most areas in southern New England. But the persisent wind and cold of February, without snow cover, damaged or killed a lot of plants that are normally reliable in zones 5 or 6. I have even seen dead zone 4 Arborovitae! So, don't believe anything you have been told about zone drift.

We are in southern Maine, right over the border. This area used to be zone 5, and with the new chart is zone 6. I still tend to stick with zone 5 when selecting plants, but have chosen a few zone 6 and am putting them in a protected area (our land tends to be windy).

The new maps are based on 15 years of data, not just a few winters. Only time will tell how accurate they are.

I can remember the rare -20 day when I was a kid, growing up in the Merrimack Valley area of Mass. That never happens now. We haven't gone below -5 at our place in Maine in 3 years. The lack of snow cover is our issue.

It does not matter that the new hardiness map is based on 15 years of data. My point is that the basis of the map that plant hardiness is a strict function of only the minimum winter temperature is faulty. The rare -20 morning in the outlying areas of southern New England only occur with radiational cooling (clear skies, calm winds, snow cover). The last time that occurred was after the big blizzard of January 1995. That would be rated as a tough winter on plants according to the hardiness map. But which is worse? -20 for one morning with snow cover and calm winds, or a week of windy mornings with temperatures just above zero and little or snow cover? I am in the process of collecting climate data and developing a new map that integrates duration and frequency of cold as well as wind and duration and depth of snow cover. This scheme would also work better as a way to keep track of zone drift if any, in response to climate change.

I completely understand this very good point about duration and frequency of cold temperatures. But, if a plant can survive a cold temperature of say zero, I didn't realize that this meant maybe only for one or two days of the year (your Missisissippi example). Isn't that a plant classification issue more than a zoning issue? If the plant is classified to be able to withstand zero degree temperatures, but can only survive one or two days of them, shouldn't it really be reclassified? What are your thoughts?

I agree with rockman.

Sue, 15 years IS only a few years, when you're talking about this issue. Surely, you must vacation in FL for most of the winter if you believe that -20 days in New England "never happens now". Granted, your winters probably are a bit milder, maybe even putting you a zone ahead of me, here in central NH, but only last winter we had more than a handful of -20 days, even one stretch that lasted almost a week if I recall, getting down to even -27.

Yes..I guess I am saying that plants would have to be reclassified with different zone designations based on their ability to withstand cold temperatures for different amounts of time. For example, a Crape Myrtle might be able to live though one brief exposure to -5 degrees for a few hours, before a warm up. But it might die if that same temperature is reached on numerous occassions during winter.

I think the whole zone classification thing is an oversimplification, in terms of reflecting what plants will grow where. Besides the fact that it doesn't reflect duration of cold spells, it ignores other weather factors like average winter precipitation levels and prevailing winds, which play an enormous role in plant survival. It's still useful, though, IMHO, as long as it's not taken for more than it's worth.

>You may be sorry, IMO, if you start getting too adventurous with pushing zones.

True. And, you may be rewarded, occasionally, by a beautiful show from a plant that nobody recognizes. I *love* it when that happens, and it makes it worthwhile, for me, to occasionally lose something that wasn't supposed to be able to survive in my area anyway. Much less painful than losing a rock-hardy specimen, too.

The New York Botanical Garden just had a 3 day symposium on "Gardening in a Changing Climate". And on April 19 and May 3("Feeling Warmth, Subtropical Plants Move North") the NYTimes had interesting articles on the effect of climate change on gardening. These articles are archived on the web, not available unless you pay for them, or find a hard copy at your library.

Apparently the effects are being felt in the South more than here: some gardeners there are giving up on Rhodies, for example. That falls in line with what Rockman says about the duration of cold spells in the south. But they will be having longer heat and drought.

It looks like the average temps have been highest since 2000, looking at the NOAA chart. That would mean the 15 year average for the zone maps may not reflect warming as much as they should. But if this is a spike from causes other than global warming, who knows? Rockman, what do you think? Are your data about duration of cold weather from predictable events? Certainly it is valuable data to have, whether historical or as a predictor.

Seems clear that zones have to be used pretty generally as guides. Unless things settle, I'm happy to experiment with vegs and annuals rather than risk valuable shrubs.

The devil is in the details. If the climate becomes more extreme, with a higher average temperature, then it might even be possible for a particular area to experience drift to a colder zone. For example, if the average summer temperature increased by 3 degrees and the average winter temperature decreased by 2 degrees, then the average annual temperature for that location would increase by 1 degree (global warming). This kind of change would wreak havoc with zone drift. I have just started to scratch the surface in terms of actual historical evidence for these types of seasonal swings in temprature, so I'm not sure yet. But, as an example, there is good data for our area that suggests December has warmed significantly (+0.2 degrees/decade over the last 100 years). But the year-to-year swing in average temperature for December has also increased (more extreme). So, since 1989 we have had one of the coldest months in history (Dec. 1989) and have also recorded some of the warmest December weather in history on numerous occassions. This is my big worry.

>This is my big worry.

The latest and greatest worry is that near-term trends (last 100 years) are out the window because we seem to have disrupted the carbon cycle - intimately tied to climate. The ocean's capacity to absorb CO2 seems to be hanging in the balance, and is gone in at least parts of the globe.

Whether this will cause a significant acceleration in the increases in greenhouse gasses and consequent temperature changes - possibly far beyond current predictions - is soon to be seen.

The link below is to one report; there are several web pages that report on the same study and on other types of ocean carbon studies. Interesting stuff!

Here is a link that might be useful: carbon saturation art. at newscientist