Myles:

FIRST ...

What are hardiness zones?

It seems every gardening book and nursery catalog refers to plant hardiness zones, also known as climate zones or growing zones. If you're new to gardening, you may be wondering what all the fuss is with these zones, and how to find out which zone you are gardening in.

Basically, plant hardiness zones are a guide to help you know which plants will grow where you live, so you don't plant things that will soon die just because they can't manage your region's temperatures. Plants vary in the temperature extremes they can endure. Basic laboratory testing can determine the lowest sustained temperature a particular plant type can withstand, but, as gardeners, we still need to know how these measurements relate to our own gardens.

USDA Hardiness Zones

In an attempt to answer this question, years ago botanists and horticulturists started gathering weather records throughout North America to compile a database to show the average coldest temperatures for each region. These records were condensed into a range of temperatures and transformed into various zones of plant hardiness. Maps were then made to show the lines between these temperature zones.

The climactic studies and maps were undertaken by two independent groups: The Arnold Arboretum of Harvard University in Cambridge, Massachusetts, and the United States Department of Agriculture (USDA) in Washington, D.C. The two maps reflected some variances, but in recent years, the differences between the Arnold Arboretum and the USDA have narrowed. Today, the USDA map, which was last updated and released in 1990 (based on weather records from 1974-1986), is generally considered the standard measure of plant hardiness throughout much of the United States. Hence we have the USDA Plant Hardiness Zones.

So what's wrong with plant hardiness zones?

Well, just think about this: The average minimum temperature is not the only factor in figuring out whether a plant will survive in your garden. Soil types, rainfall, daytime temperatures, day length, wind, humidity and heat also play their roles. For example, although both Austin, Texas and Portland, Oregon are in the same zone (8), the local climates are dramatically different. Even within a city, a street, or a spot protected by a warm wall in your own garden, there may be microclimates that affect how plants grow. The zones are a good starting point, but you still need to determine for yourself what will and won't work in your garden.

How many zones are there?

The USDA plant hardiness map divides North America into 11 hardiness zones. Zone 1 is the coldest; zone 11 is the warmest, a tropical area found only in Hawaii and southernmost Florida (and maybe the very southwest corner of San Diego County, California). In between, the zones follow a fairly predictable pattern across the continent, though a closer look will reveal scattered patterns of variations. Generally, the colder zones are found at higher latitudes and higher elevations.

Applying zone references

Plant encyclopedias may refer simply, for example, to "Zone 6," which generally means that the plant is hardy to that zone (and will endure winters there), and generally can withstand all the warmer zones below. More detailed information may indicate a range of zones (i.e., "Zones 4-9"), which means the plant will only grow in those zones, and will not tolerate the colder and warmer extremes outside them. But remember, zones are only a guide. You may find microclimates that allow you to grow more than the books say you can; by the same token, you may find to your dismay that some precious plant -- one that's "supposed" to be hardy in your zone -- finds its way to plant heaven instead.

If you live outside North America

You can roughly translate the USDA hardiness zones by finding out how low your area's temperatures can reach, and then use the chart below to find your corresponding zone.
Zone 1: below -46 C (below -50 F)
Zone 2: -46 to -40 C (-50 to -40 F)
Zone 3: -40 to -34 C (-40 to -30 F)
Zone 4: -34 to -29 C (-30 to -20 F)
Zone 5: -29 to -23 C (-20 to -10 F)
Zone 6: -23 to -18 C (-10 to 0 F)
Zone 7: -18 to -12 C (0 to 10 F)
Zone 8: -12 to -7 C (10 to 20 F)
Zone 9: -7 to -1 C (20 to 30 F)
Zone 10: -1 to 4 C (30 to 40 F)
Zone 11: above 4 C (above 40 F)

Sunset Zones versus USDA Zones

Gardeners in the western United States sometimes are confused when confronted with the 11 Hardiness Zones created by the USDA (United States Department of Agriculture), because they are used to a 24-zone climate system created 40 years ago by Sunset Magazine. The Sunset zone maps, which cover 13 Western states, are much more precise than the USDA's, since they factor in not only winter minimum temperatures, but also summer highs, lengths of growing seasons, humidity, and rainfall patterns to provide a more accurate picture of what will grow there.

If you live in the western U.S., you'll find that nurseries, garden centers, and other western gardeners usually refer to the Sunset climate zones rather than the USDA plant hardiness zones. In fact, the Sunset zones and maps are what are listed for each plant in Sunset's Western Garden Book and Western Garden CD-ROM, which are considered the standard gardening references in the West.

However, the USDA zones are still of importance to western gardeners, since the USDA zones are used in the rest of the country. When you order plants from catalogs or read general garden books, you need to know your USDA zone in order to be able to interpret references correctly.

SECOND...

The AHS Plant Heat Zone Map
by H. Marc Cathey, AHS President Emeritus

Most gardeners are familiar with the U.S. Department of Agriculture's Plant Hardiness Zone Map. By using the map to find the zone in which you live, you will be able to determine what plants will "winter over" in your garden and survive for many years. That map was first published in 1960 and updated in 1990. Today nearly all American references books, nursery catalogs, and gardening magazines describe plants using USDA Zones.

But as we all know, cold isn't the only factor determining whether our plants will survive and thrive. Particularly during seasons of drought, we are all aware of the impact that heat has on our plants. And although there is still disagreement in the scientific community on this issue, many believe that our planet is becoming hotter because of changes in its atmosphere.

The effects of heat damage are more subtle than those of extreme cold, which will kill a plant instantly. Heat damage can first appear in many different parts of the plant: Flower buds may wither, leaves may droop or become more attractive to insects, chlorophyll may disappear so that leaves appear white or brown, or roots may cease growing. Plant death from heat is slow and lingering. The plant may survive in a stunted or chlorotic state for several years. When desiccation reaches a high enough level, the enzymes that control growth are deactivated and the plant dies.

Using the Heat-Zone Map

Use the AHS Plant Heat-Zone Map in the same way that you do the Hardiness Map. Start by finding your town or city on the map. The larger versions of the map have county outlines that may help you do this.

The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days).
Thousands of garden plants have now been coded for heat tolerance, with more to come in the near future. You will see the heat zone designations joining hardiness zone designations in garden centers, references books, and catalogs.

On each plant, there will be four numbers. For example, a tulip may be 3-8, 8-1. If you live in USDA Zone 7 and AHS Zone 7, you will know that you can leave tulips outdoors in your garden year-round. An ageratum may be 10-11, 12-1. It can withstand summer heat throughout the United States, but will over winter only in our warmest zones. An English wallflower may be 5-8, 6-1. It is relatively cold hardy, but can't tolerate extreme summer heat.

Gardeners categorize plants using such tags as "annual" or "perennial," "temperate" or "tropical," but these tags can obscure rather than illuminate our understanding of exactly how plants sense and use the growth-regulating stimuli sent by their environment.
Many of the plants that we consider annuals-such as the petunia, coleus, snapdragon, and vinca-are capable of living for years in a frost-free environment. The Heat Map will differ from the Hardiness Map in assigning codes to "annuals," including vegetables and herbs, and ultimately field crops as well.

Plants vary in their ability to withstand heat, not only from species to species but even among individual plants of the same species! Unusual seasons-fewer or more hot days than normal-will invariably affect results in your garden. And even more than with the hardiness zones, we expect gardeners to find that many plants will survive outside their designated heat zone. This is because so many other factors complicate a plant's reaction to heat.

Most important, the AHS Plant Heat-Zone ratings assume that adequate water is supplied to the roots of the plant at all times. The accuracy of the zone coding can be substantially distorted by a lack of water, even for a brief period in the life of the plant.

Although some plants are naturally more drought tolerant than others, horticulture by definition means growing plants in a protected, artificial environment where stresses are different than in nature. No plant can survive becoming completely dessicated. Heat damage is always linked to an insufficient amount of water being available to the plant. Herbaceous plants are 80 to 90 percent water, and woody plants are about 50 percent water. Plant tissues must contain enough water to keep their cells turgid and to sustain the plant's processes of chemical and energy transport.

Watering directly at the roots of a plant-through drip irrigation for instance-conserves water that would be lost to evaporation or runoff during overhead watering. In addition, plants take in water more efficiently when it is applied to their roots rather than their leaves. Mulching will also help conserve water.

There are other factors that can cause stress to plants and skew the heat-zone rating. Some of them are more controllable than others.

BOTTOM LINE [per me]...

Use the Sunset Western Garden Book climate zoning system for most of your gardening needs. Use the USDA system when you're talking gardening and plants with horticultural friends outside the West. (Although there is a Sunset "National Gardening Book" that divides the entire U.S. into 45 climate zones [including our 24]).

Joe