Know Your Zone

Will dahlias grow well in my yard? When is the best time of year to plant a new cypress tree? If I leave my annuals in containers on the deck in November will they survive? Will chrysanthemums flourish in my garden? When can I plant vegetable seedlings in my garden?

Gardeners have many questions when it comes to selecting and caring for plants, trees and shrubs. Asking those important questions will increase gardening success. There are many factors to consider before purchasing plants and planting in your garden. The first step is to know your climate.

The United States Department of Agriculture has made this easy by putting together a color-coded map of the United States, based on the average annual minimum temperature for each zone. This map is called the Hardiness Zone Map. It is the map that most gardeners in the United State look to for guidance on planting. Your local nursery or garden center probably sells a majority of the plants that flourish in your zone. In fact, many plant tags will indicate “ideal growing zone”. If you are unsure whether a plant will flourish in your zone, consult your local nursery or garden center staff.

History of the Hardiness Zone Map

Although the original Hardiness Zone Map was created in 1960, the current map is the version from 1990. This 1990 version shows in detail the lowest temperatures that can be expected each year in the United States (and Canada and Mexico). These temperatures are referred to as “average annual minimum temperatures” and are based on the lowest temperatures recorded for each of the years 1974 to 1986 in the United States and Canada and 1971 to 1984 in Mexico. The map shows 10 different zones, each of which represents an area of winter hardiness for the plants of agriculture and our natural landscape. It also introduces zone 11 to represent areas that have average annual minimum temperatures above 40 F (4.4 C) and that are therefore essentially frost free.

How the map started

Every plant can adapt to a range of environments. Gardeners have learned through experience where the great variety of landscape plants can be grown. Over the years many schemes have been proposed to help gardeners locate those environments when they introduce new species, forms, and cultivars. In 1960, the pooling of many of these schemes culminated in the development of the widely used “Plant Hardiness Zone Map,” under the supervision of Henry T. Skinner, the second director of the U.S. National Arboretum. In cooperation with the American Horticultural Society, he worked with horticultural scientists throughout the United States to incorporate pertinent horticultural and meteorological information into the map.

Why the new map was created in 1990

The “Plant Hardiness Zone Map” was published in 1960 and revised in 1965. Since then, many changes, new interests, and new responsibilities have emerged in North American landscaping. One of the most apparent changes was in the weather. We have been losing plants, from our landscapes, which 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. An additional interest was in the introduction of new forms of traditional plants that are adapted to a wider range of environments than the older forms. Finally, horticultural scientists determined that due to our common borders with Canada and Mexico, we share indigenous plants and they expanded the scope to include the whole of North America.

The development of the 1990 map

The average annual minimum temperature data were analyzed for Mexico, the United States, and Canada. Of 14,500 stations that measured temperature during the period of interest, almost 8,000 could be identified by latitude and longitude and by a valid average annual minimum temperature (i.e., an average based on at least 10 years of data). Data from only the latter stations were used in the map. The data were archived by Servicio Meteorologico Nacional (Tucubaya, D.F., Mexico), the National Climatic Data Center (Asheville, NC), and Environment Canada – Canadian Climate Centre (Downsville, Ontario). Temperature data were compiled and maps prepared under contract with the Meteorological Evaluation Services Co., Inc., 165 Broadway, Amityville, New York 11701. The map is an Albers Equal Area Projection. Standard parallels of 29.5 , and 45.5 , were used to generate the map of the three countries. The map was computer generated by latitude and longitude. Because of the large area involved, it is not possible to draw one map that is accurate for all of North America. The part representing the United States has the least distortion. The Agricultural Research Service proposes to periodically evaluate weather data and issue updated maps as necessary and appropriate.

How to use the Hardiness Zone Map

Zones 2-10 in the map have been subdivided into light- and dark-colored sections (a and b) that represent 5 F (2.8 C) differences within the 10 F (5.6 C) zone. The light color of each zone represents the colder section; the dark color, the warmer section. Zone 11 represents any area where the average annual minimum temperature is above 40 F (4.4 C). The map shows 20 latitude and longitude lines. Areas above an arbitrary elevation are traditionally considered unsuitable for plant cropping and do not bear appropriate zone designations. There are also island zones that, because of elevation differences, are warmer or cooler than the surrounding areas and are given a different zone designation. Note that many large urban areas carry a warmer zone designation than the surrounding countryside. The map-contains as much detail as possible, considering the vast amount of data on which it is based and its size.