New Hardiness Zones on the Way



Remember the big news about the USDA rejecting the revised zone map it commissioned the American Hort Society 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. 

Let's hope so.


  1. This is timely! I have been questioning the current map since it doesn’t seem to apply anymore. I thought the map from the Arbor Day Foundation was more accurate. What surprised me and shouldn’t have is that the map is of concern to gardening companies.
    Of course, I believe that local gardening authorities should set local maps as well so that between the USDA map and the local map we could have better information.

  2. The gardeners who quit gardening when plants die are certainly not out there reading hardiness zone maps! (No, they’re in the garden center at my workplace, looking for maintenance-free gorgeous color for dry shade.)

  3. I thought Chicago was 5a with 5b starting half way down state. There must be a very small area bordering Lake Michigan in the city.

  4. Glad there is a new map that meets the logic test (assuming it does). There will always be small areas that don’t quite work with the general map so I think the very detailed ones that get into microclimate areas are a good idea. We have the bandwidth so why not, I say! Still, the overall general maps need to reflect the majority of an area so there will always be people who wonder why things don’t work the same for them.

  5. Not only does Lake Michigan have a tempering effect on the weather, but the City has the heat-island effect, although the green roofs are an attempt to undo some of that heat island effect. The suburbs to the north of the City along the Lake are Zone 5b, as are those to the south, but the Northwest and Western ‘burbs are solidly 5a, we’re the ones who had the -21 a few years back, and -18 to -20 this past winter. I hope that clears things up.

  6. Nice that the new map is supposedly more accurate, but just because a plant is rated hardy (or not) in the zone you’re supposed to be in doesn’t necessarily mean it’s going to survive (or die) in your garden.

    There could be other factors affecting the microclimate — soil diseases or pests, or your own particular gardening habits could be unsuited to the plant’s needs.

    Or, for a -21 degree winter, there could be a nice thick 3-foot snow cover that keeps things alive.

    Frankly, I’m sick of hearing how businesses need scientific information to reflect things they want consumers to hear, rather than actual data.

    I’m in Zone 5b, and Zone 6 plants are surviving in my garden just fine.

Comments are closed.