High Elevation Gardening Web Log "Blog"

In response to requests for better plant hardiness guidelines, the hardiness map has been updated, not by the USDA but rather the National Arbor Day Foundation. This new map is based on the most recent 15 years of data from 5,000 climate data stations across the United States.

The ‘2006 arborday.org Hardiness Zone Map’ reflects what has been suspected; many areas have become warmer since 1990 when the last USDA hardiness zone map was published. Most of Flagstaff is now classified as a balmy zone 6. This means that the average minimum low temperature is between 0 and minus 10 degrees. Lower elevation areas in Coconino County are now labeled zone 7 with minimum winter temperatures not dropping below zero.

The new map is viewable at www.arborday.org/media/Zones.cfm. You can type in your zip code to find your zone, down load a high resolution map, and see a map that highlights zone changes from 1990 to 2006. The latter map shows that over half the United States is at least one zone warmer. The new map is not surprising as the ten warmest years on record have occurred since 1990.

Zone 6 may seem like good news to many gardeners as this zone brings many more plant options. Who wouldn’t want to successfully grow a smoketree, hydrangea or a musk rose? Many plants grow well in a range of hardiness zones so higher temperatures will likely not affect many of our popular ornamental species such as maple, crabapple, spirea, and lilac.
But warmer temperatures may also mean drier times and with water a limited resource, selecting an appropriate plant should be based on more than just hardiness.

The goal of the new hardiness zone map is not to speculate why the climate has warmed but rather to provide gardeners with better information for tree selection. The National Arbor Day Foundation supports planting trees and they view tree planting as a positive action that people can take to counteract global warming.

Should we change our plant selections based on the new map? When selecting perennials and smaller shrubs, experiment and see what works. Utilizing the microclimates on your property may even allow you to dip in the vast list of zone 7 plants.

It’s different for trees and larger shrubs. These plants are long-lived, more valuable and have a greater place in the landscape. It may take only one below normal weekend to cause considerable damage or even kill a plant that has been around for decades. Being zone conservation is wise when choosing trees and large shrubs, especially for those that provide structure to your landscape.

The USDA is currently revising their map using a 30-year data set. They plan to include other factors that influence plant success such as humidity and heat adaptation.
Knowing that a plant's performance is governed by many factors including length of growing season, timing and amount of rainfall, elevation, wind patterns, the influence of large bodies of water, summer highs, and humidity, Sunset Garden has incorporated all of these factors in their complex climate zone map. Their map is available on the web at www.sunset.com/garden, and updated zone information can be found in their recently released 8th edition of the ‘Western Garden Book.’ Unfortunately, few nurseries and plant guides utilize these helpful garden climate zones.

By Hattie Braun
The author is the Master Gardener program coordinator for Coconino County Cooperative Extension. If you have a gardening question, call the Master Gardener hotline at 774-1868 ext. 19 or visit our Web site: highelevationgardening.arizona.edu.