Garden Zones

Gardeners have come to rely on USDA Hardiness Zone Maps to tell them what plants will survive in their climates. New garden zone maps will be coming out shortly with greater detail and hopefully accuracy.

What Are Hardiness Zone Maps?
In the mid-1900s, the U.S. Department of Agriculture (USDA) mapped out the entire United States, Mexico and Canada by lowest annual minimum temperature groupings. Each garden zone represented a 10 degree F. difference. This was invaluable advice for the agriculture industry. Now plants could be rated by hardiness garden zones, taking the guesswork out of choosing plant varieties. You had a gage, other than experience, for picking plants.

The garden zone maps have been revised over the years, to reflect changes in climate. When cities and towns were moved from one garden zone to another, gardeners were left to wonder what would happen to their existing garden plants.

While our climate may be shifting, these changes did not occur over night. Plants are adaptable, surviving in many different climates. They also cannot read gardening zone maps.

In 1990, the garden zones were further divided, with each numbered garden zone being broken down into an ‘a’, the lower temperature end of the garden zone, and a ‘b’, the higher. Unfortunately plant breeders have yet to start using these distinctions, so they are mostly useful if a gardener wants to push the envelope a bit. A gardener in garden zone 6b will be tempted to dabble in 7a plants. Given the variability of climate, it’s a hit and miss situation.

The American Horticulture Society (AHS) introduced a Plant Heat-Zone Map in 1997, intended to supplement the hardiness map. Heat related problems are much harder to quantify. High summer temperatures only told half the story. Plants don’t usually react to a day or two of heat they way they might respond to a frost. However, plants subjected to a two week heat wave could well succumb. Other variables that weren’t considered were things like humidity levels, nighttime temperatures and rainfall.

What’s Changing with Garden Zones?
The AHS was awarded a grant to update the USDA Hardiness Zone Map. They studied 30 years of weather data and are in the process of updating the garden zone map to include mitigating circumstances such as the length of cold spells in the winter, airflow patterns, the effect of large bodies of water like oceans and lakes and heat factors. The distinction of ‘a’ and ‘b’ sub-zones is gone. There will now be 15 garden zones instead of the current 11.

How Will This Affect My Garden?
Hardiness garden zones are more of a tool for gardeners than gospel. Weather is just to unpredictable. What’s growing well in your garden now should continue to grow well. The usefulness of the new Hardiness zones will mostly depend on how well plants are tested and labeled. How quickly the plant industry will adapt to using these new garden zone maps remains to be seen, but it will make it more difficult for people like me, in zone denial, to come up with excuses for high plant mortality. For more gardening related articles click here

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