Thursday, November 29th, 2007...4:46 pm

Hardiness Temperature Zones: What Zone Will Make My Plant Happy?


TheGardenLady has been writing about a hardinessTemperature Zone Map for use when planning and planting your gardens. For those readers who wonder what TheGardenLady is writing about, let me explain.

There are a number of factors/needs that are involved in growing plants. For example, each plant has its special water needs, special soil needs or special fertilizer needs, etc. to allow it to grow to its optimum best. One of the important factors in raising plants outdoors is the temperature plants need to allow them to metabolize properly.  Plants grow best within a range of temperatures; for some plants the temperature range will be narrow and for others the range can be wider. Plants also differ in their ability to survive frost.

How does a gardener know what temperature range they live in to best grow the plants they want? There is something called a plant Hardiness Zone Map. Most places in the world have this information.  Plant Hardiness Zone maps for around the world  can be found here. 

The Plant Hardiness Zone Map most commonly used in the US can be found here.  This map was created in  1990 based on the lowest and highest temperatures recorded from 1971 to 1984 in the US, Mexico and Canada.

A newer map just recently posted by the Arborday Society uses newer temperature figures based on the changes in weather from 1990 to 2006.  The older US map is still useful but climate changes are having an effect of warming in these zones – to enable plants from warmer climates to grow or not grow in your area.

As shown on the Arborday Temperature Zone Map now certain areas can try to grow plants that originally grew only in warmer zones.  At Triple Oaks Nursery in central New Jersey, for example, they are trying to “push the envelope” and have an outdoor all year show garden where they grow more exotic plants, plants that had never survived the winter before in their area.

How many zones are there?  The USDA plant hardiness map divides North America into 11 hardiness zones. Zone 1 is the coldest; zone 11 is the warmest, a tropical area found only in Hawaii and southernmost Florida. In between, the zones follow a fairly predictable pattern across the continent, though a closer look will reveal scattered patterns of variations.  Generally, the colder zones are found at higher latitudes and higher elevations. To read the map, you find your Zone and then when you look at plant information, the material will tell you which zone/s the plant is hardy to ( to endure winters). Most information will indicate a range of zones where the plant will grow but will not tolerate the colder and warmer extremes outside them.

Sunset Zones versus USDA Zones

Gardeners in the western United States may be confused when confronted with the 11 Hardiness Zones created by the USDA (United States Department of Agriculture),  because they are used to a 24-zone climate system created approximately 40 years ago by Sunset Magazine.  The Sunset zone maps, which cover 13 Western states, are much more precise than the USDA’s, since they factor in not only winter minimum temperatures, but also summer highs, lengths of growing seasons, humidity, and rainfall patterns to provide a more accurate picture of what will grow there.

If you live in the western U.S., you’ll find that gardeners in your area  usually refer to the Sunset climate zones rather than the USDA plant hardiness zones. In fact, the Sunset zones and maps are what are listed for each plant in Sunset’s Western Garden Book and Western Garden CD-ROM, which are considered the standard gardening references in the West. However, the USDA zones are still of importance to western gardeners, since the USDA zones are used in the rest of the country.

Of course, I reiterate that other factors determine whether plants will survive to grow in your garden.   Soil types, rainfall, daytime temperatures, day length, wind, humidity, microclimates and heat also play their roles.  But consider the plant hardiness zone map an important beginning when checking out new plants that you want to get for your garden.

If you live outside North America and can’t find your temperature zone but know the lowest temperature and highest temperature in their area, you can attempt to do some conversions with the US Hardiness Temp Zone Map.  You can roughly translate the USDA hardiness zones by finding out how low your area’s temperatures can reach, and then use the chart below to find your corresponding zone.

Zone 1: below -46 C (below -50 F)
Zone 2: -46 to -40 C (-50 to -40 F)
Zone 3: -40 to -34 C (-40 to -30 F)
Zone 4: -34 to -29 C (-30 to -20 F)
Zone 5: -29 to -23 C (-20 to -10 F)
Zone 6: -23 to -18 C (-10 to 0 F)
Zone 7: -18 to -12 C (0 to 10 F)
Zone 8: -12 to -7 C (10 to 20 F)
Zone 9: -7 to -1 C (20 to 30 F)
Zone 10: -1 to 4 C (30 to 40 F)
Zone 11: above 4 C (above 40 F)

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