Understanding Zones

Gardeners may benefit from knowing which hardiness zone new plants prefer.

  • Plant hardiness zones from the USDA helps you select the plants right for your area.
    The USDA plant hardiness zones from 1990. The USDA is in the process of revising the map.

  • Plant hardiness zones from the USDA helps you select the plants right for your area.

Gardeners can be conservative or adventurous, traditional or daring. The conservative or traditional gardeners choose plants they know will grow well in their gardens. They depend on these plants for a successful garden year after year. The adventurous or daring gardeners want to push the limits and grow plants that are not commonly found in their areas. For these gardeners, success (and failure) is part of the fun of gardening. Both types of gardeners can benefit from understanding zone maps and hardiness ratings when choosing perennials that fulfill their garden aspirations and annuals that add seasonal color.

Hardiness zones or ratings are listed in gardening magazines, reference books, plant catalogs and on seed packets. 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 and Zone 11 has the warmest winter temperatures.

A plant listed as 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. 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 zones. However, there are many factors that affect a plant’s ability to grow in a particular climate including exposure, altitude, moisture, soil type and even snow cover. These conditions create variations between and within zones.

While it’s unlikely that a plant listed as hardy in Zone 7 would survive in Zone 4, you may be able to grow plants recommended for warmer zones. Your garden may have a microclimate that’s not typical of the actual zone where you live. Variations in temperature, moisture, soil and light can be used to place plants where they have the best chance of growing. Trees and buildings provide shade and protection from strong winds, while south-facing fences and walls absorb heat to create a warmer microclimate.

You may give plants special attention to help them survive out of their recommended zone. For example, gardeners living in Zone 5 might be able to grow a Zone 6 or even a Zone 7 plant if their gardens are in protected locations or they provide extra care such as growing plants in a warm south-facing garden and the plants are covered with mulch or snow during the winter. Another option is to move plants into an unheated garage or another protected location that doesn’t get as cold as the outdoor temperature. Some gardeners may even move tender plants indoors during the winter months.

While cold temperatures can damage plants, extreme periods of heat can injure plants while they are actively growing during the summer time. In 1997, the American Horticultural Society developed a Heat Zone map that divided 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.

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