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Don’t worry—we’re not starting a new Throwback Tuesday series. But after reading Thomas Christopher’s post detailing his issues with the current USDA plant hardiness map, I felt it would be useful to revisit previous Rant discussions on this highly controversial topic. There have been plenty. The mere fact that it took over 10 years to come up with an acceptable revision of the old 1990 map gave us all plenty of time to debate it.

For example, in 2007, I visited Tony Avent at an open house at Plant Delights, his fabulous nursery and test garden, located just outside of Raleigh. He was in the middle of working on the map revision. Here’s an excerpt from that post:

Avent, who has been involved with the map revision for some years, explained the goof that was made with the first attempt in 2003. “That map was not accurate. It got rid of half the zones [the a’s and b’s]. It was easier to use, but it was wrong. Chicago would have been zone 6.” After that first map (which Avent says was created by a consultant the USDA sent off to make a map so he’d “stop bugging them”), the UDSA called together a new committee, and decided on a 30-year average of temps rather than a 20-year average, which would have created too dramatic a shift. Actually, you can read about much of this in the Plant Delights catalog and website. (I do think there are going to be a lot of disappointed gardeners regardless of how they do the changes. Sure, some plants will be hardy where once they were not. Some winters.)
Avent is always one for the long view and showed me a petrified palm found on his property (shown above) that he says dates from the cretaceous era, about 90 million years ago—this to help illustrate his view that periods of warming come and go. He’s not too concerned about global warming, and to be honest I wasn’t that concerned about discussing it with him. I have my views, and he obviously has his—but I was there for the plants.

Then, in 2009, Susan Harris wrote about the initial rejection of the 2003 revision and the process toward a “revised revision:”

Remember the big news about the USDA rejecting the revised zone map it commissioned the American HorticulturalSociety to do for it?  And then it was tweaked a bit and released by the Arbor Day Foundation.  And of course the USDA denied that the new map was rejected for political reasons.
Well, the USDA’s just about ready to release a revision they DO approve of, as reported in the Daily Climate.
In that article Tony Avent is quoted criticizing the rejected map because it moved the zones too far southward—apparently moving Chicago from Zone 5b to Zone 6.  Says Avent: “In 2004, Chicago had a  -21º winter.  If Chicago gardeners had planted zone 6 plants, they would all have failed.  When plants die customers give up gardening, and that’s the nursery business’s worst nightmare.”
And another expert backs up that opinion: Michael Dosmann, curator of living collections at Harvard University’s Arnold Arboretum in Boston and an advisor on the USDA revision, expects that the new map will be much more credible than either the AHS version or the current
1990 map.  “I think we can have a lot of faith in it.

Finally, in 2012, after the revision was released, Michele Owens interviewed David W. Wolfe, Cornell plant scientist and expert on the impacts of climate change on agriculture.

Owens: 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?
Wolfe:  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.

The fact is that, while it’s true that a 30-year perspective “dilutes” change, it’s also true that if I get more winters in Buffalo like we had in 2013-14, there’s no way that zone 6 plants will survive in my yard. If there’s any conclusion to draw, it may be this: rather than calling for new zone maps, climate change makes zone maps more and more irrelevant.

Posted by

Elizabeth Licata
on March 22, 2016 at 8:00 am, in the category Gardening on the Planet, Ministry of Controversy.


  1. Chicago in zone 6 is quite a laugh. Even half way to St. Louis still isn’t a zone 6 even if this past winter we had no temperatures below zero F. And while you all are on about climate maps, too many cold hardy alpine plants cannot handle our hot summer temperatures combined with too little data about what summer heat zones these plants can handle.

  2. All you have to do is perform a daily google news search on “trees and climate change” to see how things are rapidly changing. My neighbor died 7 years ago. She was a gardener. A single 40s-something tech guy moved in. He neglects the yard. Countless plants have died. One large spruce died during the severe drought a few years ago. Now, during dry spells, I drag my hose and buckets of water over to his yard and water his trees and shrubs. I feed birds and attract pollinators. The birds and pollinators use his towering trees for shelter, reproduction, and food. While I live here, his neglect will be matched by my attention.

  3. Plant communities in nature and in gardens are defined more by extremes than they are by averages. Extreme winter lows as was experienced in the midwest in 2013-14 killed or severely damaged trees that were deemed hardy by the most recent USDA Hardiness Zone Map and by Michael Dirr’s assessment of some species in his Manual of Woody Landscape Plants, 2009. For example, Atlas cedar did not prove hardy in midwestern areas of USDA Zone 6a during the winter of 2013-14 as was also the case during the winter of 1993-94. In my opinion, the reluctance to radically change hardiness zones from the 1990 USDA map had nothing to do with climate change denial, and everything to do with providing a more useful map for gardeners.

  4. Trying to find a database, preferably spreadsheet of some sort, showing US zip code in one column and planting zone in another. Can anybody help? I google and usually end up at USDA, who seems happy to charge about $500 for something I’m really not sure is what I want.

  5. While I can appreciate the arguments from both sides on the zone maps, I track the temperatures in my own garden and have determined that I still live in zone 5 here in Maine. If I choose to plant one zone up (6), then it is because I have identified a micro climate within the garden and I am willing to take the chance. My own beef with the USDA is with average frost dates. Stop pushing this information. Annuals should be planted based on soil temperature and nighttime low temperature, not an arbitrary date.

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