A thriving garden isn’t created by chance—it’s the result of thoughtful planning, careful observation, and informed plant selection. One of the most important tools for gardeners, whether beginners or seasoned experts, is the plant hardiness zone. This simple classification system forms the foundation for choosing plants that will flourish in your climate, helping you avoid frustration and unnecessary expense.
What are Plant Hardiness Zones?
Plant hardiness zones are geographical areas defined by their average minimum winter temperature. The most widely used system in North America is the U.S. Department of Agriculture (USDA) Plant Hardiness Zone Map, which divides regions into zones numbered from 1 (the coldest) to 13 (the warmest). Each zone represents a 10-degree Fahrenheit (F) difference in average annual minimum temperature and is further divided into “a” and “b” subzones for more precision.
Red Baneberry
Hardiness Zones: 5b 6a 6b 7a 7b
Learn more about Red Baneberry
Why Hardiness Zones Matter in Garden Planning
The primary role of hardiness zones is to identify a plant’s ability to survive cold weather. If a plant is labeled “hardy to Zone 5,” it means it can endure that zone’s typical minimum temperatures. Choosing plants outside your zone’s range can lead to dieback, reduced blooming, or complete plant loss.
Hardiness zones also help guide plant placement. For example, a gardener might grow a borderline-hardy shrub in a sheltered microclimate—near a south-facing wall or protected courtyard—where the temperature tends to stay slightly warmer than the surrounding area.
Beyond Cold: Limitations of Hardiness Zones
Although extremely useful, hardiness zones don’t tell the whole story. They measure cold tolerance but do not account for:
- Heat tolerance
- Humidity levels
- Soil type and drainage
- Seasonal rainfall
- Sun exposure
- Wind patterns
For instance, to address heat tolerance, the American Horticultural Society created a Heat Zone Map that measures the number of “heat days” above 86°F. A plant might be hardy to Zone 5 but intolerant of high summer temperatures in Zone 9. Successful garden planning means considering both ends of the temperature spectrum.
Common Milkweed
Hardiness Zones: 5b 6a 6b 7a 7b
Learn more about Common Milkweed
Using Hardiness Zones in Your Garden Designs
When you know your zone, you can plan more confidently and create gardens that thrive naturally.
- Select the Right Plants
Look for zone ratings on seed packets, nursery tags, plant catalogs, online listings, or on the Brandywine Native Garden Hub. Favor plants whose hardiness range includes your zone or is one step below, allowing room for colder-than-average winters. - Experiment Within Reason
Gardeners in warmer zones may push boundaries by trying plants rated for slightly cooler zones, especially in protected microclimates. For example, with all their concrete and blacktop holding heat longer than nearby rural areas, cities are “urban heat islands” and may be a zone above the surrounding landscape. Also, lower elevations tend to be warmer than higher ones; therefore, gardeners in cold climates might consider growing tender plants in containers and overwintering them indoors. - Design for Resilience
Zone-appropriate plants tend to be lower maintenance, more resistant to local pests, and better adapted to natural rainfall patterns. This results in healthier gardens and reduced need for intervention.
Find Your Hardiness Zone
The USDA updated its national plant hardiness zone map in 2023 using the average annual coldest temperatures from 1991 through 2020. This is the first update since 2012, which used data for 1976–2005, and reflects the changing climate. For example, Chadds Ford, Pennsylvania has seen an increase of 3°F. The USDA states that the new map “is generally about one quarter-zone warmer” than reported in the 2012 map, and “Zones 12 and 13 have been introduced for regions with average annual extreme minimum temperatures above 50 degrees and 60 degrees F, respectively.”1 You have to live in Puerto Rico or Hawaii to plant these zones!
Find your hardiness zone by entering your zip code at this website, which has an interactive GIS-based map. Remember, zones are an approximation; consider your microclimate.
Conclusion
Plant hardiness zones are an essential guide for any gardener aiming to create a sustainable, thriving landscape. While not the only factor to consider, they provide a crucial starting point for choosing plants that will survive and flourish in your climate. By understanding your zone and combining that knowledge with thoughtful design, you set the stage for a garden that grows beautifully year after year.
— Written by Melissa Reckner, Penguin Court Program Manager, Brandywine Conservancy
Resources
1 USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture. Accessed from https://planthardiness.ars.usda.gov/pages/map-creation.