Wednesday, July 6, 2011

Hardiness Zones, Heat Zones, and Sunset Zones

USDA Hardiness Zones

The USDA (United States Department of Agriculture) has been producing a Hardiness Zone Map for many years.  This map divides the U.S. into Zones 1 through 10 based on minimum temperatures (1 being the coldest and 10 being the warmest).  A plant is placed into one of these Hardiness Zones based upon the lowest temperature it can withstand.  Over the years, the concept of Hardiness Zones have been applied to all areas in the world.

2006 National Arbor Day Foundation Hardiness Zone Map

The most recent USDA Hardiness Zone Map is from 1990.  However through work from the American Horticulture Society, the Arbor Day Foundation has created the 2006 Hardiness Zone Map.  This takes into account the warmer temperatures the U.S. has been experiencing over the last decade.  If you are interested in seeing the changes, the Arbor Day Foundation has a great animation on this page. Just click the Play button. 

Probably the most useful site for locating your Hardiness Zone is located here.  Just enter your zip code and your Hardiness Zone will be shown.

The benefit of Hardiness Zones is that it provides a starting point for planning which plants can winter-over where you live.  However, there are a few drawbacks to the Hardiness Zone Map.  It does not consider day length (changes considerably the further from the equator you go), snow cover (moderates soil freezing and insulates roots), humidity, frost, or soil moisture.  Probably the biggest drawback is that it does not consider how warm your summer will be.  The classic example is comparing the Shetland Islands north of Scotland and southern Alabama.  Both are listed as bewteen Hardiness Zone 8-9.  However, the Shetland Islands are sub-artic and southern Alabama is sub-tropical.  There are almost no plants that can grow in both places.

AHS Plant Heat Zone Map

The variations in summer heat around the globe is what prompted the American Horticultural Society to create their Plant Heat Zone Map.  The AHS Plant Heat Zones are listed from 1 through 12.  The zones are based on the average number of days per year that the temperature will rise above 86 degrees F (30 C) as this is the temperature above which plants start to show heat stress.  This is a great tool to augment the Hardiness Zones in your planning.  You can dowload a PDF of the AHS Plant Heat Zones here.  You can also search for your Heat Zone on this page by entering your zip code.

Sunset Climate Map

The last map that I want to share was produced by Sunset Magazine.  This company, which has a large gardening focus and has been around since 1898, divided the U.S. into 45 Climate Zones.  These zones are based on Latitude, Elevation, Ocean Influence, Continental Air Influence, Mountains, Hills, Valleys, and Microclimates.  It is a much more ambitious undertaking.

Here is a searchable map to find your Sunset Climate Zone, but I prefer going to the Sunset website and selecting the U.S. region here, as it provides a map as well as information on that growing region. Sunset also provides a pretty substantial searchable plant database for their Climate Zones on this page

Combining Hardiness Zones, AHS Plant Heat Zones, and Sunset Zones, you will be able to more confidently chose plants that are well suited to your local conditions.  These are great tools for planning your Permaculture System.

Here is a link to my article on Hardiness Zones for the World.

For my reference, I also want to add a Pacific Northwest Hardiness Zone Map as well.  It is based on the 1990 data, but is still useful.

4 comments:

  1. How does zone D (Kitsap county) relate to the USDA hardiness zones I wonder?

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  2. The link mentioned here for ascertaining heat zone by zip code takes you to the American Horticultural Society's web page, with no place to enter a zip code. In fact, a search of the site turns up "no results" for "zip code." If I ever find a way to find my heat zone by zip code I will put a comment here to let everyone know. So far all my internet searches have not turned up a way to do this. :(

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  3. AHS will sell you a map but there doesn't seem to be a way to get your zone without buying their map.

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  4. You can find Heat Zome maps by state here.... http://gardeningplaces.com/heatzonemap/

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