The Brand New USDA Hardiness Map: Already Out of Date?


UntitledMeet David W. Wolfe, Cornell plant scientist and expert on the impacts of climate change on agriculture. Wolfe is also the author of an absolutely delightful book about the soil, Tales from the Underground: A Natural History of Subterranean Life, that ought to be required reading for all gardeners.

In the 2011 Timber Press title The New American Landscape: Leading Voices on the Future of Sustainable Gardening, Wolfe wrote the chapter on gardening and climate change. This week, I called him up to ask about the new USDA hardiness zone map, after I looked up my own zone and got a result that in no way corresponds with current reality. Zone 5a?  Maybe 20 years ago, but not now. (For more about the map, see Elizabeth's post here.)

Q: The old zone map was based on a 13-year average of winter minimum temperatures from 1974 to 1986. The current map is based on a 30-year average that reaches all the way back to 1976. In a time of rapid winter warming, how does looking at this longer time scale make any sense?

A:  First, hats off to the USDA for doing a new map.  We haven't had an official one since 1990 and a lot of nurseries and commercial growers have been waiting a long time for a new map. Why the USDA chose a 30-year average, I don't know. It means we can't really compare it to the past map.  

And since the spans overlap, a good percentage of the data in the new map was already in the old map. This dampens the perception of change.

Of course, there is the argument that looking at a bigger swath of time is more reliable. On the other hand, we know we are on a trajectory, so a longer time span will dilute more recent changes. A 15 year-map would have been preferable in my opinion.

That said, the map still shows a significant shift towards higher average minimum winter temperatures in the Northeast and Ohio, as well as other places.  The map corroborates changes we are seeing in the living world, as plants bloom earlier, as insects appear earlier, as their ranges move northwards.

For conservative gardeners, this map is fine.

Adventuresome gardeners, on the other hand, would probably prefer something else. In fact, in 2002, the New York Botanical Garden planted a test garden in the Bronx of plants hardy only to a zone warmer than its zone on the 1990 map. These plants are all doing fine.

Q: Though I'm delighted that warming winters make similar experiments plausible in my own yard, your chapter in The New American Landscape makes it clear that climate change is not all fun and games.  We're all probably aware that warmer winters mean new pests, but you mention other effects, such as "de-hardening" during winter warm spells.

A: Well, when you say "pests," your readers probably think of insects, but warmer winters are also really great for deer. Without a long period of snow cover when they can't feed, they have better survival rates, produce more young, and are likely to want more food from your garden.  

About "de-hardening": The buds and stems of perennial plants go through a physiological hardening process so they are protected from winter damage. If they "de-harden" during a warm spell, then the threshold for winter damage is at a higher temperature. In the winter of 2003-2004, we had a very warm December in upstate New York and then cold temperatures in January. It caused millions of dollars' worth of damage to new plantings of European wine grapes  It wiped them out down to the ground. In late winter, a warm period can cause a premature leafing out and frost damage.

Many perennials also have a chilling requirement that may not be met in future, so they may not be able to produce a good amount of flowers and fruit. Apples, for example, have a long winter chilling requirement: 1200 hours with temperatures below 45 degrees. So do our native Concord grapes. On the other hand, European wine grapes, Vitis vinifera, have much less of a chilling requirement.  We may be growing more of those.

Q:  In The New American Landscape, you recommend "cautious exploration" with less hardy plants on the part of gardeners.  Why not wild experimentation? 

A: Actually, gardeners can lead the way here, figuring out how we can take advantage of the opportunities offered by a warming climate, because it's not their entire livelihood at stake, as with farmers.  Maybe we need VIctory Gardens in a new context, that of climate change.


  1. The wild cultivated garden is being set up for some de-hardening damage as we speak, a potential repeat of the horrors of 2007. My elevation may provide a slight buffer compared to down below. Now you just wait and hope the higher freezing temperature damage threshold doesn’t show up in this winter that wasn’t.

    Now let’s see how long it takes for the climate change deniers to arrive to spread confusion.

  2. I’m a little concerned about the lack of cold this year—trying to grow poke milkweed by sowing seed directly outdoors in fall. A normal year it’d get enough cold to trigger germination, but we’re getting seventy-degree days in January, and I’m not sure if I’ll get anything coming up at all.

    I’ve already moved into semi-spring prep. We could still get a harsh cold snap, and I’m not gonna plant out tomatoes, but it definitely feels like we skipped winter entirely this year.

  3. I too have been watching my garden with great concern this “winter”. We have seen warmer weather than most April/May months of my life. A high of 9 Celcius the other day is just nuts! I have run races in May when it has been snowing, that should be expected, but 9 in January is not good.

  4. It’s been ridiculously warm here (Northern California) this winter, despite some night-time lows well past freezing. Today’s high ? 65 or more. But the warmer weather seems to coincide with lack of moisture, too. We are at 40% or less of historical average rainfall. Snowfall in the mountains is even worse, and that’s where out summer water comes from. The only bonus in this weather change (not gonna call it climate change just yet – still seeing it as an anomaly for now) has been that the past two summers were much cooler than expected. If this warmer winter weather does take hold, the good news is it’ll be warm enough I can grow avocados & bananas in my backyard; the bad news is that there’ll be no water for them.

  5. You of little attention spans……2010-11 was the coldest in forty years in the Northeast. Yet you clima-latte’ drinkers can only worry about right now. And this year Alaska is hosting record cold……………

    But it is now called climate change so no matter what happens they win and you keep drinking the clima-latte’s

    The TROLL

  6. Dear Troll: Do some research. What is systematic about climate change is EXTREMES of all kinds. Cold as well as heat.

  7. You have drunk the Clima-Tea! What happened to Global cooling then global warming…….
    They call it climate change for the simple reason that no matter what happens they are right and people like you fall right in line.

    The TROLL

  8. Why the USDA chose a 30-year average, I don’t know. It means we can’t really compare it to the past map.

    Dr. Wolfe should have perhaps been asked to be on that team instead of, say, Tony Avent (and I love Plant Delights). Here’s what Tony’s e-letter said:

    On August 18, 2004, USDA formed a technical review committee of 23 people including yours truly…..Several of us had pushed for a 30-year map, which would more closely echo short term natural temperature fluctuations, and the USDA agreed.

    It seems to me that a longer term averaging of temps would do the opposite of “echoing short term natural fluctuations” but perhaps there was something lacking in my 30 years in the scientific community that would explain my inability to understand that sentence.

  9. The problem I have with all this comes at plant-ordering time each spring. For example, Burpee’s has an annual that I really want, so I ordered it and requested a mid-May ship date (for several reasons, but mainly because here in western NY, nobody with sense plants annuals outdoors until that time. Well, Burpee’s refused – the response was basically that mid-April is when we ship to your zone. Would that I actually did live in a zone where I could plant annuals outdoors in mid-April……….

  10. Is there someway to crunch the data and show a 15 year average? I think that would be valuable so conservative planters could use the 30-year map, and adventurous gardeners could use the 15 year map.
    Certainly agricultural universities, urban planners, landscape designers, and forestry services would be interested. Not to mention those home gardener who live right on the cusp of 2 zones, as I do in Atlanta.

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