What are Heat Zones?
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 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. Based on numerous studies, the consensus of scientists is that our planet's climate is warming 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.
Plant cells require oxygen for respiration. Either too much or too little water can cut off the oxygen supply to the roots and lead to a toxic situation. You can control the amount of oxygen your plant roots receive by making sure your plants have good aeration-adequate space between soil particles.
Light affects plants in two ways. First, it is essential for photosynthesis-providing the energy to split water molecules, take up and fix carbon dioxide, and synthesize the building blocks for growth and development. Light also creates heat. Light from the entire spectrum can enter a living body, but only rays with shorter wavelengths can exit. The energy absorbed affects the temperature of the plant. Cloud cover, moisture in the air, and the ozone layer-factors we gardeners can't control-affect light and temperature. But you can adjust light by choosing to situate your plant in dappled shade, for instance, if you are in its southernmost recommended heat zone.
Daylength is a critical factor in regulating vegetative growth, flower initiation and development, and the induction of dormancy. The long days of summer add substantially to the potential for heat to have a profound effect on plant survival. In herbaceous perennials and many woody species, there is a strong interaction between temperature and daylength. This is not a controllable factor in most home gardening situations.
While a gentle spring breeze can "cool" a plant through transpiration as it does us, fast-moving air on a hot day can have a negative effect, rapidly dehydrating it. Air movement in a garden is affected by natural features such as proximity to bodies of water and the presence of surrounding vegetation, as well as structures such as buildings and roads. You can reduce air circulation by erecting fences and planting hedges.
If the environment is wooded, transpiration from trees and shrubs will cool the air. On the other hand, structures of brick, stone, glass, concrete, plastic, or wood will emit heat and raise the air temperature. Gardeners wanting plants to produce early or survive in cold zones will often plant them on the south side of a brick wall. Obviously, this would not be a good place for a plant at the southern limit of its heat zone!
The ability of plant roots to take up water and nutrients depends on the relative alkalinity or acidity of the soil. Most plants prefer a soil close to neutral (pH 7), but there are many exceptions, such as members of the heath family, which prefer acidic soil. The successful cultivation of any plant requires that it be grown in a medium within a specific pH range. While it is possible to manipulate the pH of soil with amendments, it is easier to choose plants appropriate to your soil type.
Plants vary greatly in the ratio and form of elements they need for consistent, healthy growth. When these are present in appropriate quantities, they are recycled over and over again as the residue of woody material and dropped leaves accumulates and decays, creating sustainable landscapes.
How the map was created
The data used to create the map were obtained from the archives of the National Climatic Data Center. From these archives, Meteorological Evaluation Services Co., Inc., in Amityville, New York-which was also involved in the creation of the Hardiness Map-compiled and analyzed National Weather Service (NWS) daily high temperatures recorded between 1974 and 1995. Within the contiguous 48 states, only NWS stations that recorded maximum daily temperatures for at least 12 years were included. (Due to the amount of missing data in Alaska and Hawaii, the 12-year requirement was reduced to seven years at several stations.)
Because they were too difficult to map, data from weather stations at or near mountain peaks in sparsely populated areas were not incorporated. A total of 7,831 weather stations were processed; 4,745 were used in plotting the map.