The United States Department of Agriculture separates the US into 13 climate-based growing zones (called hardiness zones) to determine which plants thrive in different areas.
Some of the terminology associated with gardening and plants may be confusing to those who are new to gardening. For example, it may be necessary to provide an explanation of the USDA zone. This is an important system for identifying the plants that will thrive and grow in specific regions of North America.
Your are likely to have a thriving garden when planning in advance and understand how these hardiness zones work.
Hardiness Zones: What Do They Mean?
Every few years, the U.S. Department of Agriculture (USDA) generates and updates the USDA plant hardiness map. North America is divided into eleven zones based on the minimal average annual temperatures. Lowering the number results in a decrease in the temperature of the zone.
A temperature difference of ten degrees is represented by each zone. Each zone is further divided into “a” and “b” segments. Each of these represents a five-degree temperature difference. For example, zone 7 represents minimum temperatures between 10 to 20 degrees F. The a and b subdivisions represent 10 to 15° degrees F. and 15 to 20° degrees F.
Hardiness as a term refers to how well a plant can survive cold temperatures. Nevertheless, the USDA zones fall short in that they neglect to consider other variables. These comprise the effects of snow cover, precipitation, and elevation, as well as freeze dates and freeze-thaw cycles.
Why Are Hardiness Zones Important?
Gardeners must consider hardiness zones when growing plants because not all plants are able to thrive in every type of climate.
Simply put, temperature is the ultimate driver of plant life. When temperatures fall below freezing, plants cease to grow. Annual plants shrivel up and die, while perennial plants pull all of their energy and resources inward and prepare for the winter dormant season.
For example, in Florida, tropical fruit trees keep producing bananas, avocados, and mangoes during the mild winters. On the other hand, in Alaska, the temperature can dip to -50 degrees Fahrenheit, and the only fruit crops available are the cold-hardy ones such as the Siberian kiwi.
In a broad sense, your planting zone determines the varieties of plants that can endure a typical winter in your area.
How to Use Hardiness Zone Information
Understanding hardiness zones means you can choose plants for your garden that are most likely to survive the winters in your area.
The zones are not that important for annuals since these are plants you would only expect to survive through one growing season. However, remember to check the USDA zones for perennials such as trees, and shrubs prior to planting them in your garden.
The limitations of the USDA zones are particularly apparent in the western part of the U.S. If you live in this area, you may want to use the Sunset climate zones. This system uses more than just the minimum temperatures to determine which plants grow best where. Additionally, they use the duration of the growing season, summer temperatures, wind, humidity, and rainfall.
Please take into consideration that no classification and/or zoning system is perfect. Even within your own garden you may have important microclimates that impact how plants grow. Refer the USDA or Sunset zones as a guide and consistently check them to give you the best chance of success in your garden.
Suggested Reads:
- USDA Planting Zones: What Is My Growing Zone?
- Tips & Advice about Gardening by Zone in the US
- Heat Zone Map Info: What Do Heat Zones Mean
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