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January 11, 2006

Going to extremes

About twenty years ago I had a chance to chat with Henry Marc Cathey, then-director of the US National Arboretum in Washintgon, DC. He had come into the nursery where I worked, I recognized him, and I asked him a few questions about a new project he'd embarked on with the USDA.

That new project was an updating of the USDA's Plant Hardiness Zone map. A revered resource for American gardeners, especially east of the Rockies, the Hardiness Zone map charted average low winter temperatures across the country. In those days, it was at best a general guide, a rainbow flag of broad horizontal stripes - isotherms, or areas of similar temperature - across the US. Dr. Cathey grew animated as he described the process of fine-tuning the map. Nowadays, people might refer to his task as "increasing the fine granularity" of the thing, making it potentially more useful to gardeners and farmers. Microclimates can shift a low temperature upward or downward by a good ten degrees, depending on your exposure, elevation, proximity to large bodies of water, and so forth.

Cathey expected it would take several years to complete, and it did. Our conversation took place in, I think, 1985. It was 1990 before the map was released. I'd moved back to California at that point, and was moving out of the landscaping business, and so seeing press reports of the map's release brought a sort of frisson of nostalgia and not much else.

Out here in the Big Ol' West gardeners don't use the USDA hardiness map much. Average winter low temperatures are important in determining whether or not a plant will grow in a particular spot, but that's just the beginning. Average summer highs are crucial as well, as are the amount of precipitation and when it falls, humidity, wind, soil conditions, groundwater conditions and a few other things. Coos Bay Oregon, Fresno California, and Tucson Arizona were all, even on the 1990 map, located in the same hardiness zone - Zone 9, with average lows between 30 and 20 degrees F, but a very different set of gardening rules characterizes each of those towns. Western gardeners tend to use other guidelines, such as Sunset Magazine's climate zones, which take other climatic factors into account.

The USDA map had so much become part of my dim distant past, I hadn't heard that it had again been revised significantly to account for changes in US climate over the last two decades until today, when Amanda emailed me a link to a post on Diane Dees blog. The update, published in 2005 by the National Arbor Day Foundation, is chilling - despite the evidence that most of the country is getting warmer.

Here's a graphic I yanked from the Arbor Day Foundation's site:

hardiness map update

You can click on the image to see it at much greater resolution on my Flicker site. The bottom two maps are the 1990 climate zones, on the left, and the updated one to the right. Above them is a metamap showing which parts of the country had their zones changed, by how much, and in which direction.

It is tempting to read both more and less into the maps than one should. Those tan stripes across the country in the top map likely had as much change in climate as the red or blue ones, but defining zones in multiples of ten degrees means a place that went from 21 degrees average winter low to 29 degrees will stay in the same zone and thus be colored tan, while a neighboring, warmer area with a smaller change - say from 28 to 31 degrees - will shift zones. It's startling to look at the difference in the maps between the Adirondacks, or Shenandoah National Park and see whole islands of color wink out between 1990 and 2004, and those may well be devastating changes - or they may be artifacts of the gradient itself.

But a surprising number of places have warmed by more than ten degrees, enough to bump them up two whole zones. Included are a couple stretches near North Cascades National Park, a huge swath of the Salmon River headwaters, Kalispell, the Wasatch Plateau and Glen Canyon and Los Alamos, a patch near the Straits of Mackinac, some of northeast Missouri, a spot along the Platte near Grand Island, and almost all of Cape Cod. Superficially counterintuitively, some of the country seems to have gained colder winters as a result of global warming. Most of that land is in the arid southwest, where winters are warmed by sunny weather. Boil more winter storms up out of the Pacific to blanket the desert in overcast and the temperature will drop. Winters near the west end of Grand Canyon National Park have cooled by more than ten degrees, dropping that land from Zone Eight (20° minimum) to Zone Six (0° minimum.)

All of this in 14 years.

While warmer winters in and of themselves might seem benign, many plants rely on cold winters for survival or reproductive success. For instance, balmy winters in Upstate New York may mean economic ruin for orchardists, whose apples need a certain number of chilling hours per winter to set fruit. And it takes more than average winter lows to gauge what will grow in your garden, and climate changes broad enough to so radically shift isotherms will likely have other effects as well. Warmer winters may mean either drier winters, or warmer summers, or both. Thus a shift in average winter low from 10 to 20 may correlate to drought deaths of forests, as trees struggle to thrive in warmer summers with less water. Frosts help control insect and pathogen populations, and reducing their frequency and severity may well cause irruptions of pest organisms such as bark beetles.

Unfortunately, drawing ecological conclusions from a gardener's map of average temperatures carries a special risk all its own.

Averages are useful to gardeners, because gardeners can play the odds. If I plant tomatoes on April 15, and two times out of ten a killing frost hits on May 1, I can remain perfectly happy and fulfilled, accepting a one in five risk that I'll have to run to the nursery for more plants in mid-May.

But the Grand Canyon was not carved by average rainfalls. It was carved by 100-year flash floods. The northern limit of a wild tree's range is circumscribed not by average winter lows, but by the low temperatures hit in especially cold years. 99 years in 100, the winters a hundred miles north might be just warm enough for the tree to thrive, but so what? That hundredth year will wipe them all out. The same goes for floods, summer highs, droughts. Averages are close to meaningless in the ecological realm. What shapes the wild environment around us is the extremes.

The Earth's climate is a complex, dynamic system. It cycles and fluctuates in proportion to the amount of energy put into the system. The more energy is kept in the system, the wider one can expect its fluctuations to be. Current climate change, caused by increasing the amount of CO2 and other greenhouse gases, is a result of those gases' prevention of excess solar energy from radiating out into space. That energy has to go somewhere. Storms will get larger, fronts stronger, the differences between average and extreme seasons greater.

These maps are frightening enough as it is. They are more so for the information they do not include.

Posted by Chris Clarke at January 11, 2006 01:40 PM TrackBack URL for this entry:

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Comments warming doesn't exist! It's junk science!

Seriously, here on the East Coast we live and die by the USDA map. Especially those of us trying to grow tea roses on the knife edge of zone 7, or get banana trees to flourish in a microclimate.

A few years ago someone on GardenWeb posted a link to a super-duper fine-grained version of the map that allowed you to zoom right in on your own county. Whee! We were all so excited.

Now, if someone can just explain to Jackson & Perkins that I'm not in zone 6, I've never been in zone 6, that I'm not even in zone 7 even more, because zone 7 has turned into zone 8!!! I don't even order from J&P anymore -- they refuse to ship their stock to me at the proper time, instead holding it until we're well past the frost date circa 1975.

Lovely post on global warming, Chris, and I'm sorry for going all gardener on you. I guess that zone map just does it to me.

Posted by: Violet Socks at January 11, 2006 07:08 PM
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Help! I live in a place, as you know, that straddles two zones. Which one should I plant for?

Posted by: Roxanne at January 11, 2006 07:49 PM
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That boundary you used to straddle has picked up and moved. The only part of Maryland still in Zone 6, according to the Arbor Day Foundation map, is out west of Cumberland. You're smack in the middle of Zone 7 now. Enjoy those camellias.

Posted by: Chris Clarke at January 11, 2006 07:59 PM
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Cool. Good to know. I would have been embarrassed to ask 'em over at Johnson's.

Posted by: Roxanne at January 11, 2006 08:19 PM
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There are more oaks in my grandfather's old woodlot now than there were 50 years ago. It was all red and sugar maples back then. The few black spruce are all gone too, replaced by hemlock. I'll have to get back to the (not very) high country in the Adirondacks this year and see if I can notice changes from years ago. Thanks for the great post Chris. Fits right in with my melancholia.

Posted by: OGeorge at January 12, 2006 09:54 AM
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It's funny, I was reading along, thinking, oh, that's kinda sad, oh, that's a shame, until the words "North Cascades National Park" jumped out at me, and then all of it just hit me like a sock in the gut. My life as an apartment dweller doesn't leave much room for gardening, but the mountains and the surrounding forest, now that means something to me.

Why aren't you a lobbyist?

Posted by: nina at January 12, 2006 01:38 PM
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The Arnold Arboretum in Boston has been tracking flowering times for various plants - they've been getting earlier, no surprise. This jives well with the hardiness map changes.

Posted by: Buffalo Gal at January 14, 2006 06:06 AM
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Rox, and other gardeners on the edge of two zones:

One approach is to be most conservative with trees, fairly conservative with shrubs, and try any perennial that appeals. (for reasons of cost, longevity, crucial-ness to a garden plan, etc.)

Marc Cathey was also behind a 'heat zone' map put out some years ago, but it was much less worthwhile than it could have been, relying only on daytime highs without any use of the equally important nighttime lows. That one was funded by Monrovia, not a USDA project. I wonder if it's been updated or improved.

Posted by: Nell at January 14, 2006 09:49 AM
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It was distressing to see the whole citrus zone in Central Florida surrounded by significant changes. There's only a small zone around Orlando where the temperature stays consistent enough to grow oranges. Too hot or too cold, forget about it.

Posted by: MoXmas at January 30, 2006 06:49 AM
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