Plant Hardiness Zone Maps

Every so often, a winter comes along that tests our mettle, and so far the winter of 1996 seems to be one of those occasions. Blizzards, dangerous wind-chills, and record breaking low temperatures turn even a simple trip to the grocery store into an epic battle for survival. But have you ever wondered how our landscape plants manage to survive climatic extremes. The physiology behind their ability to survive is beyond the scope of this article, however, either by chance or careful planning, most of us have chosen trees and shrubs that are actually quite comfortable in the frigid Iowa landscape. But selecting "hardy" plants doesn't have to be a game of chance. In fact, plant survival in a particular region can be predicted with a great degree of accuracy by knowing something about hardiness zones and corresponding zone ratings given to woody and herbaceous landscape plants.

Plant hardiness zone maps have been a valuable aid for many years to those interested in predicting the adaptability of plants to specific climatic areas. Most are isotherm maps of geographic regions based upon average annual minimum temperatures experienced at certain weather stations over some period of years. Zone ratings given to plants are meant to indicate excellent overall adaptability. Many plants may survive in zones warmer or colder than their designated zones, but mere survival does not necessarily represent satisfactory performance.

Many hardiness zone maps are available. Some cover small areas such as individual states, while other encompass entire countries. Unfortunately, many do not agree in their numbering schemes, and therefore zone numbers can be related only to the map used in assigning them. The most widely used map in this country is the Plant Hardiness Zone Map prepared by the Agricultural Research Service, (USDA).

The first plant hardiness zone map for the United States and Canada was developed by the USDA in 1960. Zones were numbered 1 to 10 with Zone 1, the coldest, located in sections of northern Canada, and Zone 10, the warmest, at the southern tips of Texas and Florida. Each zone is based on ten-degree Fahrenheit differences in the average annual minimum temperatures and is split into sub-zones ('a' and 'b') of 5 Fahrenheit degrees. Lower temperatures occur in the 'a' zone.

The new Plant Hardiness Zone Map was released by the USDA in 1990. As in the old map, each zone is divided into 'a' and 'b' regions based on five-degree differences. Much of the northern half of Iowa is classified as zone 4b (expected minimum temp. range -25 to -20F), while a significant portion of the southern half of Iowa is in zone 5a (expected minimum temp. range -20^o to -15^oF). It is important to remember that the temperatures shown on the Plant Hardiness Zone Map are developed by taking an average of temperatures that fall above and below the final listed temperature, and that no year will exactly fit an average. Because of this limitation, the map cannot and should not be expected to predict record low temperatures as experienced in 1996. Finally, hardiness zone boundaries should not be recognized as absolute. When close to a boundary between two zones, choose plants hardy for the colder zone. And always provide excellent care and a protected site if marginally hardy plants are used.

The new Plant Hardiness Zone Map should help professionals and homeowners make wise plant selections. But several other factors are as important as minimum winter temperatures in influencing plant survival. Presence or absence of snowcover, timing of cold weather, location of plants in the landscape, and relative health of plants as they enter winter also will play a role in how successfully plants cope with an Iowa winter.

USDA Hardiness Zones and Expected Average Annual Minimum Temperature Range

This article originally appeared in the February 9, 1996 issue, pp. 9, 1996 issue, pp. 12-13.