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HARDINESS ZONE MAPS AND PALM TREES

written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
Published 11/30/2012

Have you ever wondered if we can grow palm trees in the Tri-Cities? Do you know how to determine if a palm tree or any perennial, woody tree or shrub will survive the cold winter temperatures in our region?

The simple answer is to find out the plant’s hardiness rating and then check a ‘hardiness zone map’ to see if that plant is rated as ‘hardy’ or able to withstand the coldest temperatures experienced in ‘your zone.’ Hardiness zone maps designate zones based on the average annual minimum temperatures determined from weather data over a span of years.

A number of U.S. hardiness zone maps have been developed. The first one was produced at the Arnold Arboretum in 1927. This map divided the U.S. and southern Canada into eight zones based on 5 degree differences in the lowest mean temperature. It was the 1971 revised version of this map that I consulted when I moved to this area in 1980.

Today, the most frequently consulted hardiness zone map is the USDA Plant Hardiness Zone Map. The USDA map was first published in 1960 and revised in 1965. It divided the U.S. into zones of 10 degree differences. Updated in 1990 using more weather data, each zone was broken into two zones ‘a’ and ‘b’ with 5 degrees of difference.

You wouldn’t think that zone maps would be a point of controversy, but they have been. In 2003 the American Horticultural Society (AHS) took on the task of revising the USDA map again using more recent weather data from July 1986 to March 2002. Because there had been a period of warmer winters in the eastern U.S., some zones changed to a half-zone higher than the map released in 1990. USDA rejected the AHS map because reviewers felt the changes were taking a step backward and didn’t like that it eliminated the half zones (a&b). USDA decided to create its own map.

This year USDA finally introduced its own updated Plant Hardiness Zone Map. This map was developed by USDA

s Agricultural Research Service and Oregon State University

s PRISM Climate Group. USDA feels the new map has greater accuracy and detail, dividing the U.S. into 13 zones with each 10-degree zone divided into 5-degree zones of ‘a’ & ‘b.’

Many areas on the new map tend to be about a half-zone warmer than on the previous map and there were also some zones that are ‘cooler’ than on the 1990 map. USDA points out that this is a result of using data from a longer period of time (30 years from 1976 to 2005) where the 1990 map used data from a much shorter period (13 years from 1974 to 1986) . Changes are also the result of using more sophisticated methods of mapping and the use of temperature data from more stations.

Now back to those palm trees… plants are rated as ‘hardy’ based on the coldest winter temperatures they can survive. On the new 2012 USDA map, most of Benton and Franklin counties is designated as zone ‘7a’ with some limited colder areas of the region rated as 6b. The large palm trees found in California and Florida are generally hardy only in warmer zones of 8b to 11, making them an unwise choice for a Tri-City landscape.

Check the hardiness rating for any perennials, trees, and shrubs you want to plant in your landscape. To see the USDA map go to http://planthardiness.ars.usda.gov/PHZMWeb/# and click on Washington.

Published: 11/30/2012 2:12 PM

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