Sunny weekend of plants in Maryland
Researching a daylily

New US hardiness zone map launches soon!!

At last, the promised land is in sight!

USDA Hardiness Zone Map Image ©USDA The USDA (United States Department of Agriculture) is set to launch their new updated hardiness zone map soon. Yippee! We’ve been waiting for quite a few years now, the last update was in 1990 (left). So it’s great news that the new map, based on the latest climate data from more weather stations than before, will soon be with us. British gardeners take note: the Royal Horticultural Society is also currently looking at the whole issue of rating plant hardiness. So read on to get a sense of the issues.

So: “The Agricultural Research Service (ARS) is in the last stages of getting the map ready for release,’ Kim Kaplan of the USDA told me. “The new map is an interactive GIS-based (Geographic Information Systems) map designed to be web friendly. It will also be presented on the web as jpgs so those without broadband access will still be able to use the map.” They’re just sorting out a service to host the new map so that the vast flood of users doesn’t crash the USDA servers. And you’ll see why this has to be a web based service.

It’s going to be far more than just a printed map, with a number of positive changes. In particular, using a web interface will make it possible to see the boundaries between one zone and the next in far greater detail. The ARS has also developed a way of presenting hardiness zones effectively in areas lacking weather stations, especially in rugged parts of the Western United States.

“As a result,” says Kim Kaplan, “the mitigation of extreme low temperatures by nearby, large bodies of water will be visible for the first time, as will the presence of cold sheltered valleys and mountain tops, which are caused by elevation increases and other geographic features. The warming effect of asphalt and concrete of major urban areas will also be visible in many locations.” So, for the first time, the warming effect of cities will be reflected in the hardiness zones map. Isn’t that great?

And all this is based on the most up-to-date weather station data possible. “The 1990 map was based on weather data from 1974 to 1986,” says Kim Kaplan. “The new one will be based on weather data from 1976 through 2005. Also, more weather stations in the mountains of the western United States will be used in the new map.”

There will be no changes to the zones themselves, except an addition at the warm end. “The 1990 map had zones 1-11 a & b,” says Kim. “The new map will have zones 1-14 a & b…  Zones 12-14… have been added to the legend to allow tropical plant breeders and nurseries to provide zone information on when to bring such plants in, such as from the deck.”

The result is a map of twenty-two zones and subzones in steps of just 5F. That really is the sort of detail that all gardeners appreciate.

Musa basjoo,banana,outside Of course, this whole system can never be more than a guide. Other factors like soil drainage, exposure to cold winds are also important and snow cover. But, at last, we’re going to have a zone map that reflects recent changes in our climate. So you may find that you’re now in a warmer zone than you thought – and you can grow far more plants than you thought. Perhaps I can leave my bananas outside all winter in Pennsylvania (on the other hand…)

Have no fear: As soon as the new map goes live I’ll let you know.

This is just so good… I’m so excited… after twenty years we’re going to get a new hardiness zone map... This great… Perhaps I can grow more plants than I thought… etc etc…

(Sorry, I'm a bit lost for images considering the map's not actually out yet...)


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This is fantastic news and to have it online - wow!

Any ideas on when the RHS update might be available?

Graham Rice

Well, VegPlotter, the RHS has only recently begun to look at this issue so it will be a while, I'm afraid...

Cathy Wilkinson Barash

Any idea how soon is soon???

Christopher Lindsey


Will the GIS data be freely available for download? I seem to remember this was one of the problems that the USDA had with the earlier, more 'proprietary' maps developed by the AHS.

Graham Rice

Cathy - how soon? No fixed date. What Kim Kaplan actually said was: "The final stages mainly involve contracting to host the demand by viewers so ARS and USDA computer servers do not crash under the demand and configuring the software to run well with that host." So as soon as that's all fixed, it sounds as if the map will be online.

Graham Rice

Chris - Hmmm... that I do not know. But it certainly sounds as if it will be possible to see the map online is great detail.


I've been hearing rumors about a new hardiness zone map for some time. This is the first I've heard that it's about to become a reality.

And I might ask the same Ms. Cathy asked: Please tell me how soon is soon??


I forgot to ask how well you think it will match up with the 2006 Arbor Day Foundation zone map?

Graham Rice

TC - see my comment above about timing... looks like soon.

As to similarity to the 2006 Arbor Day Foundation map (which, as I understand it, is based on the map which had been under development on behalf of the American Horticultural Society but about which the USDA had reservations)... I suspect it will be fairly similar but in a great deal more valuable detail.

(Just as a reminder - I haven't seen the new map either!)


Do you have any more details about the Royal Horticultural society's plans?

Graham Rice

No news about the RHS plans yet, Andrea, they've only just started to consider the issue.

Blackswampgirl Kim

Oh my goodness... it seems like ALL of Ohio is now listed as Zone 6 on this map! I don't know about that--while I get away with zone 6 plants regularly up here by the lake, my friends and family in the western half of the state don't fare as well with them. I think that people who are gardening in newly designated numbers should still err on the side of caution and test things out before they buy a bunch of plants for the next warmer zone... no?

Graham Rice

You're right Kim. But let's wait till the new USDA map appears - with a and b zones at 5F intervals - and not get carried away by the Arbor Day map...

Graham Rice

John David, of the Royal Horticultural Society, sent these comments by email:

"It is fair to say that the RHS recognises the limitations of the current system of hardiness ratings, not only with respect to its applicability to the whole of the UK but also in terms of other factors which have a strong influence on whether plants may be grown in a region or not. I am, of course, aware of the views of our friends north of the border (Scotland) on this subject.

"A focus just on winter temperatures is misleading, as we know from comparing US hardiness zones with UK hardiness zones as illustrated in the European Garden Flora, for instance. Summer temperatures, which influence ripening, is also critical at higher latitudes - not to mention light levels, rainfall and soil type, all of which contribute to a plant's ability to survive a cold winter. Any system that incorporated all these would rapidly become unwieldy and therefore not of much help to gardeners.

"In the run up to the next 10-yearly review of the AGM lists in 2012, we would like to put in place a revised hardiness rating system that more adequately reflects people's experience of growing plants, while retaining the relative simplicity of the current coding system. I am open to suggestions as to what might be more useful but we will be consulting with the RHS Plant Committees for their views as well."


There is already a GIS enabled version of the usda (1990 version) at

Graham Rice

Thanks Paul, what a great resource.

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