What Are Plant Hardiness Zones?
Excerpted from: USDA Zone Explanation
When you understand how these hardiness zones work, you will be able to better plan your garden.
If you are new to gardening, you may be confused by some of the terminology associated with plants.
For instance, a United Stated Department of Agriculture (USDA) zone explanation may be necessary.
This is a useful system for determining what plants will survive and grow in certain areas of North America.
The USDA plant hardiness map is created and updated every few years by the USDA. It divides North
America into 11 zones by minimum average annual temperatures. The lower the number of
the Hardiness Zone, the lower the temperatures in that zone.
Each zone represents 10 degrees of temperature difference. Each zone is also divided into 'a' and
'b' segments. These represent five degrees of temperature difference. For example, zone 4 represents
minimum temperatures between -30 to -20 degrees F. The 'a' and 'b' subdivisions represent -30 to -25
degrees F and -25 to -20 degrees F.
Hardiness refers to how well a plant will survive cold temperatures. Where the USDA zones fall short,
however, is that they don’t account for other factors. These include freeze dates, freeze-thaw cycles,
the effects of snow cover, precipitation, and elevation.
Gardeners in Wisconsin are used to the chilly winters and, as indicated by the Wisconsin USDA
planting map above, the temperatures can dip as low as -35 degrees F.
Wisconsin has three cold hardiness zones – Zone 3 (Northwestern WI), Zone 4 (North, Central,
Western WI) and Zone 5 (Southern and eastern WI up past Green Bay and Apostle Islands).
Understanding hardiness zones means you can pick plants for your garden that will be most likely
to survive your local winters.
The zones are not important for annuals since these are plants you would only expect to survive
the summer months, or one season.
For perennials, trees, and shrubs though, be sure to check the USDA zones before you put them
in your garden.
The hardiness zone is not, and should not, be treated as the final factor in plant survival. For
example, plants experience different temperature extremes when sited in valleys, on hills, or
in urban areas. There are also multiple factors beyond minimum temperatures that determine
whether or not a plant will survive local conditions. These include average winter snow cover,
heat and humidity, and moisture and light availability.
No zoning system is perfect and even within your own garden you may have important microclimates that impact how plants grow. Use the USDA or Sunset zones as a guide
and always check them to give you the best chance of success in your garden.