by Kathleen Couch

Created on: July 30, 2010

Zone 4, Zone 6, Zone 7 to 9? What are those numbers and what are zones?  In the horticulture world the numbers and zones refer to the USDA Hardiness Zone, or planting zones.  You will find an interactive map of the USDA Hardiness Zones here.  By clicking on your area you find the planting zone number.  For a number of years, starting in the 1940’s meteorologists and the United States Department of Agriculture worked on tracking the average lows for all of the different areas in the U.S.  The zones are differentiated by 10 degrees F as the coldest in each zone. 

The zones have been revised from time to time as experience has been dictated by gardeners who have found some differences in the zone hardiness.  There are differences in some areas that seem like they should be in the same zone, because of mountains, bodies of water, or prevailing winds they are different.  The western U.S. has the most fluctuation of different zones in close areas.  Many people in the western U.S. prefer to use Zones, as published by Sunset magazine, because it is broken down in smaller areas.

Most of the United States is in Zone 4 to 8.  However Zone 11 can be found in southern California, the southern tip of Florida, and Hawaii.  Zone 1 is found in northern Alaska.  In all zones you can find microclimates due to light, elevation, winds, and water.  A microclimate can be in your backyard when something is planted in a protected area.

When you go to the local garden center, the plants will be suited to your zone.  But when you search the internet, or garden catalogs, or read a gardening book you cannot just pick your favorite flower without knowing the correct growing zone.

Growing zones with a map are used on seed packets to tell you when it is the best time range to plant.  This is the way the hardiness zone is most useful for annuals.  Most annuals will grow anywhere when it is warm enough.  In some areas they can grow all year long, but in other areas they die with the frost.  

Some plants are considered annuals in one zone, but tender perennials in another.  If you grow something that will not overwinter in your area, you can bring the potted plant indoors for the winter, or dig up the bulbs before frost.

When planting perennials, landscape shrubs, fruit trees, or shade trees the hardiness zone is of utmost importance.  These plants have to be able to tolerate the coldness of the winter in your area.

To have a garden full of thriving perennials, shrubs or trees, first find out your growing zone, and check the zone rating that is with all plant descriptions.  You will then be rewarded by an appealing landscape that continues to grow year to year.

Sources:

http://www.usna.usda.gov/Hardzone/ushzmap.html

http://landscaping.about.com/cs/lazylandscaping/g/zo ne.htm

http://www.thegardenhelper.com/hardiness.htm

Learn more about this author, Kathleen Couch.
Click here to send this author comments or questions.