A Guide to Planting Zones

By Julie Christensen

Trees, shrubs and perennials form the backbone of most gardens, and these plants are also typically the most costly. Because of their slow growth and initial cost, you expect them to last and perform well for many years.

How well plants perform, though, depends partly on the care you give them, and partly on their adaptability to your growing conditions. The closer you can match the plants’ needs to the conditions that are naturally present in your garden, the healthier plants will be with the least amount of effort.

USDA Planting Zones

One tool for choosing plants wisely is the United States Department of Agriculture plant hardiness zone map. This map is based on the average minimum winter temperatures of areas throughout the United States. Each zone is divided based on -10 degree variances. For example, the minimum temperatures for planting zone 5 range from -20 to -10. Minimum temperatures for planting zone 10 range from 30 to 40 degrees.

The plant hardiness zones don’t apply to annual flowers and vegetables because these plants are discarded at season end. Knowing your zone, though, can help you determine which perennials, trees and shrubs are likely to survive in your climate. Plants at nurseries and garden centers are labeled with their hardiness zones. Catalogs and garden books and magazines also use planting zones to aid gardeners.

Limitations

The USDA planting zones is a helpful tool, but keep in mind that it has limitations. It measures only the average minimum temperature, but doesn’t take into account altitude or humidity levels. Areas that have the same minimum winter temperature might be vastly different in other ways. Consider, for example, Austin, Texas and Portland, Oregon, both of which fall in planting zone 8. These two areas are about as different as can be in terms of summer temperatures and growing conditions.

Just because a plant grows in your planting zone doesn’t mean it will grow well in your yard. Consider how much water the plant needs. What is its native habitat? Does it tolerate wind, wet soil or drought? The cactus that thrives in Austin, Texas won’t do well in Portland, Oregon, even though both cities are in zone 8. One of the most accurate ways to determine if something will grow in your yard is simply to look around and see how common it is in your area. The plants that are used frequently in home and commercial landscapes are those that are adapted to your climate and need the least care. This doesn’t mean you can’t plant a more exotic variety, but it will probably need more attention to thrive.

Consider also, the microclimates of your yard. If you live in an urban or suburban area with lots of trees and pavement, your yard will stay warmer than that of a friend who lives in a rural area just five miles away. Rural areas stay cooler and are usually windier because there’s less pavement to trap heat and fewer homes and trees to act as wind blocks. Within your own yard, you have areas that stay warmer than others. Garden areas with a southern or western exposure are warmer than those on the northern or eastern side of your home. Areas near your home or a rock patio also tend to stay warmer. Sloped areas stay warmer than low-lying valleys, where frost pockets tend to linger.

Choosing Plants

To ensure plants that are predictably winter hardy in your area, try choosing plants that are rated as hardy one zone lower than your zone. If you live in zone 4, especially if you live in an exposed or rural location, choose plants that are hardy to zone 3. Plant questionably hardy plants in a protected area of your yard and mulch them to help regulate soil temperatures.

For more information on the USDA planting zones map, visit the following links:

USDA Plant Hardiness Map from the U.S. Department of Agriculture
Understanding the New USDA Plant Hardiness Zone Map from Greenhorn Gardening at You Tube

Julie Christensen learned about gardening on her grandfather’s farm and mother’s vegetable garden in southern Idaho. Today, she lives and gardens on the high plains of Colorado. When she’s not digging in the dirt, Julie writes about food, education, parenting and gardening.