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Zone Classifications

Cold Hardiness

Perennial plants will live longer than one year assuming they are hardy enough to withstand the winters in any specific area. For example, Daylilies are tough plants that will survive the winters in virtually all parts of North America.

Petunias are common bedding plants that Garden Centers sell in the spring as annual plants. Petunias are actually a Perennial, but they cannot survive in temperatures below freezing.

The USDA has developed a Plant Hardiness Map that divides the United States and Canada into  Zones based on an average minimum temperature. Zone 1 which is in Northern Canada has minimum temperatures exceeding -50 degrees F. while Zone 10 in Florida has a low temperature range of 30-40 degrees F.

Zone 6 with a range of -10 to zero degrees F. encompasses a wide band though the center of the country, Southern New England and much of the Pacific Northwest.

Perennial tags, catalogs and articles often include a Zone rating with the description of the plant. Rudbeckia 'Goldsturm', the popular Black-Eyed Susan, is listed as a Zone 3-9 plant. This means it will grow reliably from Central Canada through the Northern portion of Florida. Zones 1 and 2 may be too cold for the root system to survive while Zone 10 may not be cold enough to induce a needed dormancy period.

How are Zones determined for each of the thousands of Perennials available? The answer is through experience. Plants growing naturally in areas offer some information, but it is the continuous testing of plants by gardeners that eventually determines the Zone Data. Zone ratings for plants constantly change as new information is gathered and gardeners test the boundaries of plants.


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