Maybe you’ve heard of hardiness zones, but don’t really know which zone you live (and garden) in.  Or maybe you’ve never heard of even the idea of hardiness zones and want to learn more.  Or maybe you’re a gardening guru and know everything there is to know about hardiness zones…

I thought I was in the last group, but I’ve learned some new information over the past couple days whilst perusing the USDA Plant Hardiness Zone website.  It just goes to show that we can always keep learning.

hardiness map

The interactive USDA Plant Hardiness Zone Map as seen on their website.

I have a confession.  I’m kind of a map nerd, so something I really like about the USDA site is that it’s interactive.  You can type in your zip code in the top left corner and it will tell you exactly which zone you are in (I got 6a).  Or you can select a state by clicking on the map or selecting from the drop down menu above.  Here’s the map for Utah:

Utah hardiness zones

You can see that much of the Wasatch Front is on the border of zones 5b-6a, but that the state ranges from a zone 4a in Randolph to a zone 9a near St. George.

So here’s the low down on plant hardiness zones:

  • zones are based on the average annual extreme minimum winter temperature (more on that later)
  • zones range from 1-13, each sub-divided into “a” and “b” for a total of 26 categories (“1a”, “1b”, “2a”, “2b”….”13a”, “13b”)
  • they provide a general idea of which plants are most likely to thrive at a particular location based on minimum temperature requirements (there are other factors that will play into how well your plant grows, such as: light, soil, moisture, etc.)
  • in the 2012 version, zones are generally a half-zone different (usually towards the warmer side) than in the 1990 version (so if you’ve been basing your plant choices on the 1990 version, you might want to check out the updated map – it could open up more planting possibilities!)

A really important thing to remember is that first bullet in the list above; zones are based on the average annual minimum winter temperature.  For the 2012 hardiness zone map, the lowest daily minimum temperatures recorded during 1976-2005 were used to form this average.  This doesn’t mean that the temperatures will always get this low, and it also doesn’t mean that the temperatures will never drop below the listed range for the zone.  But it does give you a pretty good idea of what “normal” is.

So do the zones really matter?  Well, yes.  They let me know that I can’t expect to grow a date palm (Phoenix sp.), which are recommended to grow in zones 9-11, in Northern Utah but that I could be successful with that in St. George.  On the other hand, I could plant a red maple (Acer rubrum) which is suited for zone 6 and still not be successful due to the alkaline soil.

In other words, the hardiness zone map provides useful information about one aspect of gardening (average average annual “coldness”), but it shouldn’t be the sole resource considered when planning the garden.

 

P.S. If you’re really nerdy like me, you might want to check out the “Mapmaking” part of the USDA site to find out cool information like how data from 7,983 weather stations was used to create it.

 

Sources: USDA.gov and the Arbor Day Foundation