June 23, 2009

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2003 USDA Hardiness Map

The latest USDA Hardiness Zone Map has been updated. It now can be used in combination with the AHS (American Horticultural Society) Heat Zone Map.

So, what's the big deal? It will be very similar to the 1990 map. However, 16 years of data are incorporated. There are some other changes.

There is now a standard coding system. Plants now will be assigned four codes. Two of which will be hardiness code and two will be heat codes. The first two numbers in the pattern will be the hardiness. The first number is the coldest zone where a plant is rated and the second number will be the least cold zone in which the plant will thrive.

The second set will be the heat tolerance and requirements. The initial number of the second series will indicate the hottest zone where a plant will thrive and the last number will reflect the "least heat" zone a plant will grow.

It is the intent of the zone ratings that plants will thrive not just survive. Survival of a plant is not acceptable. Yes, plants will survive in colder or warmer zones than suggested but is that how you want to garden - by survival? Along with this is of course proper care, and important needs of the plants - proper soil type, pH requirement, water, nutrients, and correct exposure to sun and shade. A lot will still be your own experimentation where your garden micro-climates are.

The new maps will be available in the future through the US Department of Agriculture web-site. For the time being, you can digitally view it on the www.ahs.org site.

How did the new map come about? The new map was based on information collected from July 1986 to March 2002. It is interesting to note that the temperature was collected by about 7,000 weather stations some forty years ago. It was the AHS that received a grant from the ARS (Agricultural Research Service ) to create the hardness map. The AHS subsequently developed the heat zone map.


Emily's previous hardiness zone article (Zones - How important are they?) with links to European zone maps.