By Linda Naeve
The Des Moines Register

Does it seem that things are becoming more complicated than simplified?

Consider a TV remote control: It was simple to use and made it easy to turn the TV off and on, change channels, and control the volume. Now, it takes several remotes with dozens of buttons to run the TV, DVD player or VCR.

A code that uses plant hardiness and plant heat zones might seem like it's taking something simple and turning it into a complicated process. Fortunately for gardeners, the maps and codes are easy to understand, and can help us select the right plants for our environment.

Most gardeners are familiar with the U.S. Department of Agriculture's Plant Hardiness Zone Map as a standard guide to a plant's cold tolerance. It has been an important tool when selecting trees, shrubs and perennials for the landscape for nearly 40 years.

U.S. has 11 cold zones

The plant hardiness map divides the country into 11 zones based on the average annual minimum temperatures. Zone 1 is the coldest area, where minimum temperatures can go down to minus 50 degrees. Zone 11 is the warmest zone, where lows don't drop below 40 degrees. Greater Cincinnati straddles Zones 5 and 6.

Cold isn't the only factor that determines whether plants will survive and thrive in an area - heat, too, has an effect. The effects of heat damage are less obvious than that caused by extreme cold, which will either kill a plant instantly or slowly over the winter.

In 1997, the American Horticulture Society developed a new map, called the AHS Plant Heat Zone Map, to help gardeners choose plants suited to summer temperatures in their region.

This zone map, used to select vegetables, annuals, aquatic plants and turf grass species as well as perennials, divides the United States into 12 zones based on the average number of days in the year that daily high temperatures reach or exceed 86 degrees. The 86-degree point was chosen because that is the temperature at which plants begin to suffer physiological damage from heat.

The zones range from zone 1 (less than one heat day) to zone 12 (more than 210 heat days).

2 codes for cold, 2 for heat

The AHS Plant Heat Zone map is used in combination with the USDA Plant Hardiness Zone Map in a four-code system - two hardiness codes and two heat codes - that can be applied to all plants.

The first two numbers in the series are from the hardiness zone map. The first number is the coldest zone to which the plant is rated, and the second number is the "least" cold zone in which a plant will thrive. The least cold zone is important because it reflects how intense a period of "chilling" or dormancy a plant such as an apple tree or tulip needs in order to grow successfully.

The second set of numbers indicates heat tolerance and requirement. The first number indicates the hottest zone in which a plant will thrive, while the second number reflects the "least heat" zone in which the plant will grow. This is often an indication of the minimum number of warm days a plant needs to fruit or flower successfully.

On the Web: www.usna.usda.gov/Hardzone. U.S. Department of Agriculture's Plant Hardiness Zone Map; www.ahs.org/publications. American Horticulture Society Plant Heat Zone Map