I shouldn't be so surprised at the changes. Speaking only for myself here, I've known that this was coming. The map is becoming outdated for most regions of the US & Canada. These zones are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada. What are you going to do? This is 22 years ago now! I see no reason to wait for the USDA another decade or 2.

The National Arbor Day Foundation is not a government bureaucracy. However, their analysis bears more resemblance to the reality of over 100 years of data than does the USDA’s antiquated map. You can see the changes that have occurred since the USDA published the map in 1990 by clicking on PLAY.

What started me looking into all this is that on the veggie forum a gardener pointed out that zone 5 has been eliminated by Arbor Day from Washington State. I looked - that zone is also missing from northern Idaho and a part of western Montana!!

I checked the Weather Service yearly records over the last several years for some of the towns around here and altho’ Kalispell still fits within the zone 5 parameters (and is so designated by Arbor Day) the other major towns are in zone 6 and Missoula is sliding! And, it is not just the "last several years." The effects of recent warmer Winters are changing the averages for a century and more as you can see on those Weather Service pages linked above. Numbers cannot keep piling on numbers without changing averages.

And for Colorado - - well, I’ve always thought of this country up here as rather like Durango and firmly placed by the USDA in zone 5. Well guess what? Arbor Day says you are out of 5 and have gone to 6. I think you could argue both ways. Another couple places to argue both ways are in the other corner - Denver & Boulder.

Think this is all an Arbor Day and digitS' conspiracy?? Well, Colorado State University is in on it, too. Here is what they say, "The Arbor Day Map is more current than the USDA map below." BTW, you can use any US zip code on the CSU webpage to find your hardiness zone. Time for the USDA to get with redesignation.

The USDA Hardiness Zones and Average Annual Minimum Temperature Range:

4a -30 to -25 F
4b -25 to -20 F

5a -20 to -15 F
5b -15 to -10 F

6a -10 to -5 F
6b -5 to 0 F

digitS'

Well, living in Westcliffe Colorado at 9000 plus feet, I don't think that a lot of the 5/6 zone plants have a "snowballs" chance of growing here!!

We have had MANY nights this winter that were 30 to 35 degrees below zero.

I think I'll just stick with what works!!

Citydata and Wikipedia have Westcliff elevation at 7,800 feet so your home is really up there!

Too broad of a brush!

digitSteve

(I hope this doesn't turn into an exercise of finding the limitations of the Arbor Day Foundation. ;o)

In my own location, we had only one time during the last 5 years or so with the temperature dropping to -10. It was below zero a couple mornings last week but only by a few degrees. There's still some time left to the Winter, of course. But, the Weather Service no longer has any northern Idaho towns with a -10º or lower Winter minimum average except in the Bitterroots (and then not Coeur d'Alene River valley) and in the higher Palouse areas (but not Moscow). There are a few smaller towns not listed.

And I've gotta say that wind-chill just has to count . . . another limitation to all-embracing generalities. All gardening is local.

And, Pueblo had a high of 69º today! Gracious, what state is that in??

digitS'

I've listed my zone as a 4/5 on Garden Web, because most sites list my zip code as one or the other. Arbor Day is the ONLY site I've found that lists me as a 6. I'm not convinced.

Bonnie

I guess there was movement to include a 15 year run ending 5 or 6 years ago but that fell into controversy (surprise, surprise). I'd be happy if they'd just use all the data compiled for the last 100 years. That would be more reasonable than one small 12 year period - now, 2 decades off.

The answer may be at hand! I've read that Oregon State University has contracted with the gov to put together a new USDA hardiness map! And, they seem to be well on their way.

You can go to the link below and "play" with their map tool. Either keep clicking on the map until it centers on an area of your interest or type in the longitude and latitude (e.g. Casper, WY is about -106.000 Long. 43.000 Lat.)

Then you can choose minimum averages. That won't give you extremes, so this isn't quite getting you to hardiness information (that is embedded in the Weather Service data linked above). But, it does provide a nice list of temperatures from 1895 to the present and will then graph those numbers.

I find the graph a bit overwhelming for that many years. It is easier just to look at the numbers, maybe doing some averaging of decades to help round off the peaks and valleys.

This OSU tool is sophisticated and makes it easy to shake out these numbers. It works for anywhere in the US but, no doubt, it will "stumble" if there's much elevation differences near the location.

I've been out shoveling snow 4 times today. Tomorrow I'll need to find a way to get some of it off the deck roof, my bracing with 2 by 4's no longer looks adequate - too much wet snow the last 24 hours.

digitS'

My daughter lives 9 miles out of town, and the weather difference is like night and day. We get a foot of snow in town - she gets 6 feet at her house! The temp variations are the same too. Just depends on where you live.

I can see how it would be hard for the "people in the know" to figure that one out to everyone's satisfaction.

I just stick with what works. We have a nursery in Rye Colorado that sells high country plants trees and shrubs. The ladies there are VERY knowledgable about what works and was does not. That is about the only place I shop for anything new that I want to add to my garden.

So for planting purposes, I would suggest everyone just locate a nursery that has been around for a while, and ask before you buy.

Due to trial and loss - I don't put much faith in the zone info anymore.

Anyway, it's obvious that we are getting warmer here, but maybe not a firm Z5 yet. And, really, I guess I can't hope for Z5 and just ignore all the environmental implications that come with that - nor can I ignore the way the high temps are increasing also. Very unpleasant to hit 90 in early June and stay there all summer.

Right now we are at 40. The wind and warm weather has scoured away our snow cover, but I believe we'll see digit's snow tonight. It has been a very white winter!