Understanding Planting Zones

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If you have ever studied the purchase of most any outdoor plant, you will run across numbers and the term “zones”.  These are crucial to get right for long term success of plants in your landscape.

Planting zones have been developed to guide us on plants that will work in our average annual minimum temperature.  Knowing the expected minimum temperature and what plants would live through.

Most of Angelina County is in Zone 8b.  Residents of Pollock, however, are located in zone 8a.

The initial hardiness zone maps for the United States were developed during the 1920’s and 30’s. The first USDA Map was published in 1960 (USDA Miscellaneous Publication No. 814 “Plant Hardiness Zone Map.”) and was developed by Henry Skinner while he was the Director of the U.S. National Arboretum. The map showed ten broad hardiness zones based on 10 degree F. gradients.

From the beginning and for many years to come, we were simply listed in zone 8, having our annual average minimum temperatures between 10 and 20 degrees F.

During this period the Arnold Arboretum also published a Hardiness Zone Map. Donald Wyman of the Arnold Arboretum published the first version in 1938.There are subtle differences between the Arnold Arboretum and USDA maps (including the use of different temperature ranges), which lead to some confusion for gardeners.

The last map from the Arnold Arboretum came out in 1978.  By 1990, their version had fallen into disuse and the USDA version became the primary source.

In 1990 a major overhaul of the USDA map was completed by H. Marc Cathey (USDA Miscellaneous Publication No. 1475) using temperature data from 1974 to 1986. One new zone was added to coincide with adding Mexico and Canada to the map. In addition, the prior 10 degree gradients were broken down into 5 degree “a” and “b” zones.

This 1990 change in maps with “a” or “b” zones left most of Angelina County in zone 8b with average minimum temperatures between 15 and 20 degrees F.  Only the northern reaches of Angelina County are in zone 8a with expected average minimum temperatures between 10 and 25 degrees F.

One of the primary reasons given for the update was that, “We have been losing from our landscapes plants that apparently survived the 1940’s to the 1960’s.  Many of the hardiness zone classifications of plants are no longer considered valid.  In North America, the ranges of temperature and moisture for the past decade were wider than those recorded for the 1940’s through the 1960’s.” This is an intriguing statement, since it presages the current debate over recent global warming.

Does politics and plants ever collide?  Absolutely.

In 2002 the USDA initiated a project with the American Horticultural Society (AHS) to update the 1990 map. A year later the AHS released a draft of the update which showed that many of the hardiness zones had moved northward reflecting a general warming trend.

The USDA, after a brief review, decided to reject the draft and gave little justification for its decision. Its terse dismissal raised the possibility that it was a political decision – the USDA was concerned that the AHS Draft gave support to global warming proponents and decided to suppress the project.

Ten years later the USDA released the 2012 Hardiness Zone Map which added two new climate zones and showed a general warming trend across the country.  However even during the Obama Administration, the USDA noted that, “Changes in zones are not reliable evidence of whether there has been global warming.”

Regardless of anyone’s thoughts on global warming, the Angelina County Master Gardeners will be have a plant sale on Saturday, April 2 at the Angelina County Farmers Market.  They have selected plants that are well adapted to our area, they have been very successful in zones 8a & 8b, and will add tremendously to your landscape.  Gates open at 8 am and the sale continues until sold out.  At the last plant sale, plants were gone by 10:30 am.


Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu.

Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.

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