U.S. Planting Zone Guides

Hardiness zones inform you of whether you should plant citrus trees or holly bushes.

When looking for plants to populate a garden or give a landscape a boost, the issue of hardiness zones comes up quite often. If you're a beginner gardener, or if you've been at it for a while but need some guidance, understanding the zone guidelines may be confusing. However, understanding which plants work for your specific zone helps you create a thriving garden or landscape with less of a worry about plant survival.

Zone Characteristics

Zones divide the country into parts based upon their average annual low temperature. By using the lowest temperature, gardeners can choose plants based upon their chill requirements. Each zone from two through 10 is made up of a part A and a part B. Zones one and 11 lack this division. This split represents subtle temperature differences between the northern and southern part of the zone. Northern zone temperatures reach cooler yearly lows than the southern part of the zone. In addition to using the winter temperatures in zone distinction, other environmental issues come into play. The landscape and soil type, for instance, can affect the type of plants that can be grown within a zone. Other factors include humidity, moisture content, sunlight and wind.

Temperatures

Each zone covers approximately ten degrees Fahrenheit. The coldest hardiness zone in the United States, zone one represents areas that regularly reach below -50 degrees Fahrenheit each winter. In zone two, temperatures range from -50 degrees to -40 degrees Fahrenheit. Zone three covers -40 degrees to -30 degrees Fahrenheit. The temperatures for zone four range from -30 degrees Fahrenheit to -20 degrees Fahrenheit. The temperatures continue to be divided in sections of 10 degrees Fahrenheit, concluding with zone 11. In this zone, temperatures reach 40 degrees Fahrenheit and above during an average winter. The zone divisions split the temperature differences in half. For example, zone 8a covers 10 to 15 degrees Fahrenheit, while zone 8b represents 15 to 20 degrees Fahrenheit.

Sample Cities

The continental United States includes hardiness zones two through 10. Alaska and Hawaii are included in zones one and 11. Zone one, mainly made up of Canadian areas, also includes Fairbanks, Alaska. Zone two features more areas of Alaska and Minnesota around Lake Superior, including areas like Pinecreek, Minnesota, and Prudhoe Bay, Alaska. Zone three includes cities such as Duluth, Minnesota; St. Michael, Alaska; and Sidney, Montana. Cities for zone four include Minneapolis/St. Paul, Minnesota; Waterloo, Iowa; Billings, Montana; and Montpelier, Vermont.

Zone five extends through Des Moines, Iowa, and Chicago, Illinois, while zone six includes St. Louis, Missouri, and Branson, Missouri. Cities in zone seven include Little Rock, Arkansas; Richmond, Virginia; and Memphis, Tennessee. Dallas, Texas, sits within zone eight, along with Gainesville, Florida. In zone nine, cities include Houston, Texas, and New Orleans, Louisiana. Growers in Los Angeles, California; Miami, Florida; and Tampa, Florida belong in zone 10. The United States' zone 11 is solely comprised of Hawaii.

Suggested Plants for Zones

Within each zone, the plants should be able to withstand the average cold temperature. Tropical plants that will grow in zone 11, for instance, would be extremely difficult to maintain within zone one. There is some leeway for some plants; however, even though a plant may grow within a given region other than the suggested planting zone doesn't mean that the plant will grow well. Zone one plants include the quaking aspen and netleaf willow, while zone 11 features plants such as birds-of-paradise and hibiscus flowers.

Florida's palm trees and the orange groves of California indicate the warmer semi-tropical types of plants for zones 9 and 10. Holly bushes, maple trees, cedar trees and boxwood represent zone 6 and 7 plants. Zone 8 plants include fuchsia and the California pepper tree. The colder climates of zones 2 through 4 make them suitable for plants such as the shrubby cinquefoil, Japanese yew and Russian olive tree. In zone 5, growers choose from plants such as the flowering dogwood, Japanese maple and winter creeper.

References

• United States National Arboretum: Additional Information
• United States National Arboretum: Hardiness Zone Details
• United States National Arboretum: Indicator Plant Examples

Resources

• The United States National Arboretum: USDA Plant Hardiness Zone Map