Q: I am receiving a lot of nursery catalogues and all of them end the plant description by giving a zone designation. What does this mean?

A: The “zone” designation in nursery catalogues refers to “USDA Plant Hardiness Zones.” The Web site is www.usna.usda.gov/Hardzone/ushzmap.html .

There are 10 planting zones in the United States and each zone indicates a 10 degree Fahrenheit difference in the average minimum yearly temperature. A quick look at the hardiness zone map shows that elevation and distance from the equator affect the temperatures in the U.S.

Warmer areas have higher numbers. Southern Florida is in zone 10, where tropical plants can be grown outdoors, year-round.

Colder areas have lower numbers. Northern parts of Montana, North Dakota and Minnesota are in the coldest zone in the U.S., zone 3.

To add a little more precision to the zone designations, each zone is divided into 5 degree increments, “a” or “b.” Colorado Springs is in zone 5b. Black Forest is in zone 5a. Teller County, like most of our mountains, is in zone 4.

But there are other important factors that affect our growing season. 40 years ago Sunset Magazine developed zone maps which cover Colorado and 12 other Western states. The Sunset map zones use a more precise 24-zone climate system that shows maximum and minimum temperatures, length of the growing season, humidity and rainfall patterns. Colorado Springs is in Sunset map zone 1, which is considered to be an area with the coldest winters. Their Web site is: www.sunset.com/garden/climate-zones/ .

The concept of a plant hardiness map was started at the Arnold Arboretum at Harvard University. The USDA and Harvard University developed the hardiness zones during the 1920’s and 30’s, and the hardiness zone maps were first published by the US Department of Agriculture in 1960.

The map is modified every 15 years, reflecting gradual climate changes. The last map was revised by the National Arbor Day Foundation in 2006.

Most gardening reference books, nursery catalogues, gardening magazines and Extension Service publications refer to the USDA hardiness zones. However, gardeners must recognize that the map is only a guideline; it does not consider many factors necessary for plant survival.

Knowing the cold hardiness of any given plant is important, but a gardener needs to consider other factors which influence plant survival. Soil types, moisture, altitude, daytime temperatures, day length, humidity, planting location, snow cover, wind and heat are some of the factors one needs to consider. In this area gardeners often need to consider deer- and rabbit-resistant plants.

Some gardening practices may make plants more susceptible to freezing problems. If a gardener applies nitrogen fertilizer to plants after July, the plants may grow too rapidly and not adjust to freezes properly, increasing the susceptibility to frost damage.

Container plants are susceptible to having frozen roots. The ground, being dense, resists freezing, so plants survive winters much better when their roots are in the ground.

Most evergreens are well suited to Colorado, but they continuously lose moisture from their needles during the winter. Cold temperatures won’t kill them, but winters without adequate snowfall will. It is important to provide trees and bushes with supplemental water during the winter, about 5 to 10 gallons per inch of trunk diameter each month.

Contributed by Ed Carley, Colorado Master Gardener. For answers to your horticultural questions, contact the Master Gardener Help Desk at 636.8921 or CSUmg2@elpasoco.com
(A version of this article was published by The Gazette on 2/7/09.)

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