Get in the zone for gardening

July 26, 2011 - Posted by Corey Broyles | Tags: Gardening

Gardeners can be conservative or adventurous, traditional or daring.

Whatever your style, it helps to know your gardening zone, according to the National Garden Bureau, a nonprofit educational group.

The conservative or traditional gardener chooses plants they know will thrive in their garden. The adventurous or daring gardener pushes the limits, trying plants that are not commonly found in their region. Whether they succeed or fail, it’s all part of the fun of gardening.

Cold hardiness zones, or ratings, are listed on plant labels and seed packs and in gardening literature. A hardiness rating is based on a map developed by the U.S. Department of Agriculture. First published in 1960, the map divides North America into 11 hardiness zones. Each zone is determined by a 10-degree Fahrenheit difference in the average minimum temperature. Zone 1 is the coldest, Zone 11 the warmest.

A plant listed as cold hardy in Zone 4 indicates it should survive winter temperatures as low as 30 degrees below zero F. which is the average minimum winter temperature according to the USDA map. A Zone 9 plant is hardy only to 20 degrees F., according to the garden bureau.

Some references provide a range of zones in which the plant will grow. A plant listed as hardy in Zones 4-9 means it will grow in all of those.

There are, however, factors that affect a plant’s ability to grow in a particular climate — exposure, altitude, moisture, soil type and even snow cover. These conditions impact gardening between and within zones. Zone 7, for example, may be able to grow plants listed for Zone 6, maybe even 5, if your garden has microclimates that protect it. Microclimates are created when conditions change a small environment. For instance, a tree can provide shade from scorching sun, a building can shield from desiccating winds or a gutter’s downspout can create wet soil.

On the flip side, extreme heat can injure plants while they are growing during the summer time. In 1997, the American Horticultural Society developed a Heat Zone map that divides the United States into 12 heat zones. Each zone indicates the average number of days that are greater than 86 degrees F., the temperature at which plants start to suffer damage from high temperatures.

Zone 1 has the lowest number of heat days and Zone 12 has the highest number. While not used as often as the hardiness zone, it is still a valuable tool when choosing plants for your garden. The most comprehensive reference with heat zone designations is the Great Plant Guide published by the American Horticultural Society. This book lists both hardiness and heat zone information for about 3,000 plants.

Learn more about the American Horticultural Society at ahs.org and the National Garden Bureau at ngb.org.

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