The AHS Plant Heat Zone Map

Whether you are a novice or experienced gardener, you probably know about the U.S. Department of Agriculture’s Plant Hardiness Zone map. The map helps gardeners determined what species of plants thrive in the cold in specific regions of the country. The map is color-coded. To use it, simply find your particular area of the country, note the color of that region and then refer to a color-coded list of zones. The zones are numbered. Determine the zone number of your particular region and then use the number as a reference to find plants that thrives in your zone.

Light creates heat.
(Courtesy: Octubres Rotos at flickr.com)

The USDA map was first published in 1960 and has been updated twice –- in 1990 and in 2012. Today, just about every American references books, nursery catalogs and gardening magazines use the Hardiness Zone map when describing plants

With climate change becoming a major problem in every region of the United States to say nothing about its effects on the rest of the world, another map has become an essential reference when gardeners determine what plant species thrive in particular regions of the country. Scientists acknowledge that climate is warming because of climate change. So using such a map is essential in determining the best plants to grow in your particular region.

While extreme cold can kill a plant instantly, heat effects plants over time. It is slow and lingering. A plant may survive heat in a stunted or chloric state as heat damage appears in different parts. For example, flower buds may wither, leaves may droop and lure more insects, leaves turn brown or white, or roots may stop growing. The plant may survive in this state for several years. When dehydration reaches a high enough level, the enzymes that control a plant’s growth are deactivated and the plant dies.

Using The AHS Plant Heat Zone Map

A map called the AHS Plant Heat Zone Map has been created to help gardeners determine what species of plants thrive best in particular regions of the country in summer.

The map breaks the country up into 12 zones. Each zone indicates the average number of days that zone experiences temperatures exceeding 86°F. This temperature is when plants begin to suffer physiological damage. The zones range from Zone 1 in which that temperature occurs less than one day a year to zone 12 in which that temperature occurs for more than 210 days a year.

Thousands of plants have now been coded for heat tolerance. More will be coded in the very near future. The AHS Plant Heat Zones and USDA Hardiness Zones are designated together in garden centers, reference books and catalogs. Using the numbers based on your zone in combination will tell you which plants thrive outdoors in your region year ‘round.

So, for example, you plan to include tulips in your garden. If you live in zone 7 of both the USDA map and the AHS Plant Heat Zone map, you will find a score for tulips of 3-8 on the USDA map and 8-1 on the AHS Plant Heat Zone map. That indicates that tulips can thrive outdoors year ‘round. An ageratum may be 10-11, 12-1. Those numbers indicate that the plant will tolerate the summer heat anywhere in the United States, but will thrive in winter only in the warmest zones of the country. An English wallflower will receive a score of 5-8, 6-1. This means that it will do well in the cold, but won’t tolerate extreme heat during the summer.

Plants ability to tolerate heat varies from species to species as well as among individual plants of the same species. Hotter days than normal will affect results in your garden. In the case of both the Hardiness zones and the AHS Plant Heat Zone map, there will be circumstances when plants survive outside their designated zone. That’s because a number of factors are involved in a plant’s reaction to heat.

Be aware that the AHS Plant Heat-Zone ratings assume that plants are adequately watered. Lack of proper watering will adversely affect the accuracy of the zone coding. Heat damage to plants is always linked to an insufficient amount of water being supplied to the plant.

Other factors can cause stress to plants and adversely affect the heat zone rating. Some of these factors are more controllable than others. These factors include:

Oxygen
Light
Length of the day
Air movement
Surrounding structures
Soil pH
Nutrients

Oxygen

Plants require oxygen. Too much or too little water can cut off the oxygen supply to roots resulting in a toxic situation. Supplying good aeration and adequate space between soil particles can control the amount of oxygen that gets to the plant’s roots.

Light

Light is necessary for a plant to assure photosynthesis that provides energy to split water molecules, absorb carbon dioxide, and enhance growth and development. Light also creates heat. The entire spectrum of light can enter a living thing, however rays of shorter wavelengths can exit. The energy absorbed by the plant affects its temperature. Cloud cover, moisture in the air, and the ozone layer affect light and temperature. You can’t adjust that. However, you can adjust light by selecting to put plants in the shade.

Length Of Day

The length of a day regulates plant growth, flower initiation and development, and dormancy. The long days of summer adds to the potential for heat to have a major effect on a plant’s survival. In the case of herbaceous perennials and many woody plant species, there is a strong interaction between temperature and the length of the day. This is not a controllable factor for gardeners.

Air Movement

Although a gentle breeze in the spring can cool a plant, fast moving air on a hot day can cause dehydration. Natural features of a garden including the proximity of water and the presence of surrounding vegetation and structures including buildings and roads can adversely effect air movement. If desired, air circulation can be reduced with the addition of fences and hedges.

Surrounding Structures

A wooded area, trees and shrubs can assist in cooling an area. However, brick, stone, glass concrete, plastic or wood structures generate heat and help to raise the air temperature. Gardeners who want early producing plants or plants that survive in cold zones often place them on the south side of a brick wall. This is not a good location for plants at the southern limit of its heat zone.

Soil pH

The alkalinity or acidity of soil can affect the plant’s roots ability to absorb water and nutrients. Most plants do best in a neutral soil with a pH level of 7. However, there are exceptions. Plants of the heath family prefer acidic soil. Successful growing plants require a medium with a specific pH range. It is possible to manipulate the pH level of soil with amendments. However, it is easier to use plants that are appropriate for your soil.