Defining Hardiness Zones

When buying a tree or plant, you will notice an indicator on the tag of that tree or plant stating its hardiness zone. So what is a hardiness zone? It is defined as a geographic area in which a specific category of plant life is capable of growing, as defined by climatic conditions including its ability to withstand the minimum temperatures of the zone. These zones were first developed by the United States Department of Agriculture and have been updated as late as 2006 to compensate for global warming.

The USDA first issued its standardized hardiness zone map in 1960, and revised it in 1965. A new map was issued in 1990, based on U.S. and Canadian data from 1974 through 1986 (and 1971-1984 for Mexico). The new 1990 map included divided temperature zones broken into five-degree a/b zones for greater accuracy.

According to the Arbor Day Foundation, the Plant Hardiness Zones divide the United States and Canada into 11 areas based on a 10 degree Fahrenheit difference in the average annual minimum temperature. (The United States falls within Zones 2 through 10). For example, the lowest average temperature in Zone 2 is -50 to -40 degrees Fahrenheit, while the minimum average temperature in zone 10 is +30 to +40 degrees Fahrenheit. If a range of zones, for example, zones 4-9, is indicated, the tree or perennial is known to be hardy in zones 4, 5, 6, 7, 8, and 9. Suitable hardiness means a plant can be expected to grow in the zone’s temperature extremes, as determined by the lowest average annual temperature.

There are many benefits as well as drawbacks to the hardiness zone system. One drawback is that the zones do not incorporate summer heat levels into their zone determination, which means places that have extreme heat and cold could be marked for its cold zone and neglect the heat factor. Hardiness zones also do not take into account the reliability of the snow cover. Snow can act as an insulator against extreme cold, protecting the root system of hiberating plants. If snow is consistently covering the ground, it can actually lower the temperature of what the roots are exposed to.

Many factors are taken into consideration when planting trees and plants that hardiness zones do not incorporate. Some of them are: soil moisture, humidity, the number of days of frost, and the risk of a rare catastrophic cold snap.  For farmers, the probability of getting a particularly severe low temperature could be very detrimental and the knowledge of this could be more useful than just average conditions.

The last drawback of the system is the fact that although a plant will survive under particular lower temperatures, it does not mean it will flower sufficiently. For a tree to flower it requires vernalization or a particular duration of low temperatures.  Some publications are integrating such information into their hardiness zone maps. This additional information can include precipitation, wind patterns, elevation and length and structure of the growing season.

The National Arbor Day Foundation in the United States recently completed an extensive updating of U.S. Hardiness Zones in 2006. The maps include the most recent 15 years of data from more than 5,000 National Climatic Data Centers across the United States.

About the Author: Andrew Johnson is the owner of Central Texas Tree Care, a leading provider of Austin tree services in Central Texas. Certified ISA Austin arborist services including: tree trimming, tree removal, tree care and stump removal. For more information on Austin tree service please visit

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