Whether you want to plant a tree, shrub, or perennial, ornamentals or edibles, the hardiness zone of your location is the first thing to consider. Plants can often adapt to a range of conditions such as soil type or light requirements but the winter chill they are able to survive is a hard-and-fast criterion. That’s why knowing your hardiness zone is indispensable for making the right plant choices.
What is the USDA Hardiness Zone Map?
The USDA Hardiness Zone Map is based on the lowest average minimum temperatures. Using data from thousands of weather stations, it divides the country into 13 different hardiness zones. The map includes all of the contiguous United States, Alaska, Hawaii, and Puerto Rico. Canada also has a Plant Hardiness Zone Map.
The first USDA Hardiness Zone Map was published in 1960 and it is updated every few years, reflecting climate changes. The current version is the 2012 USDA Plant Hardiness Zone Map, which is based on 1976–2005 weather data. There are other hardiness zone maps that are laid out and designed differently but most of them are based on the USDA Hardiness Zone Map, which is the gold standard.
What Determines a Hardiness Zone
To divide the country into zones, the USDA Hardiness Map uses average minimum temperatures during a 30-year period. The zone numbers go by 10-degree-Fahrenheit increments. The map then further breaks down each zone into two segments, “a” and “b”, by 5-degree increments. For example, the average lowest temperature in USDA zone 7a is 0 to 5 degrees F, and 7b is 5 to 10 degrees Fahrenheit. In zone 1a, located in the coldest areas of Alaska, the lowest average winter temperature is between minus 60 and minus 55 degrees F. In zone 13a in Hawaii the lowest winter temperature is 60 to 65 degrees F.
How to Plant According to Hardiness Zones
When you buy plants or perennial seeds, check the hardiness zones on the label to make sure that it is suitable for your zone. Local nurseries usually offer plants for your zone so suitable plants have already been prescreened. For mail-order plants, you need to do the homework and identify the plants that fit your climate zone. Always keep in mind that your home might be in a different microclimate, which further narrows down or increases your plant choices (more about that below).
Note that the USDA Hardiness Zone indicates the average lowest temperature and not the lowest temperature on record. That means that if the plants you choose are just within the zone range, you are taking a slight risk. For example, if you live in zone 6a, you can plant a southern magnolia, which is suitable for hardiness zones 6 to 10; it might do well for years without any issues. However, should there ever be an extremely cold winter temperature below the average low of minus 10 to minus 5 degrees F for your zone, the magnolia might suffer cold damage or die.
What Hardiness Zones Don't Tell You
The hardiness zone where you garden is like the foundation on which you “build” your garden or landscape. Beyond that, there are several other important things to consider when selecting plants.
The microclimate of your location might be different from the surrounding area. In an urban setting, concrete and blacktop can create a hot spot in the summer and provide protection from the cold in the winter so the temperature never reaches the average lowest temperatures listed for that USDA hardiness zone.
Similarly, a home on a wind-beaten hilltop has a different microclimate than the neighbors down in the valley where it’s more protected. But living in a low-laying location can also mean you are in a frost hollow (also called a frost pocket) where cold dense air collects overnight. That can become an issue for fruit trees that have already budded or are blooming when a late frost hits. In a frost pocket, they are more likely to suffer cold damage.
Even in a small yard, you can have different microclimates. An exposed front yard is different from a sheltered garden bed along a south-facing wall, or a back yard that is protected from northern winds by a neighbor’s hedgerow.
The only way to map your own yard is by observation year-round and, unfortunately, trial and error.
Extreme Temperature Fluctuations
In the fall, perennial plants gradually prepare for dormancy. When there is a blizzard on Halloween, plants that are perfectly adapted to snowy winters could be damaged because they are simply not ready for this unseasonably cold weather, even though the temperature does not reach the minimum the plant can tolerate.
On the other end of the spectrum, a warm spell during in the winter can coax the plant into breaking dormancy. If the weather then returns to its normal cold range, the plants can suffer damage.
The hardiness zone also does not indicate the average hottest temperature, which is equally important for a plant’s health, if not its survival.
Winter temperatures cannot be evaluated in an isolate way without looking at humidity levels. The combination of winter chill and low humidity can cause more cold injury to plants than high humidity, because in high relative humidity the plant loses less moisture. That’s why cold dry winters are especially damaging for evergreens.
Average First And Last Frost Dates
The USDA hardiness zones only indicates temperatures but not when cold weather hits. The last spring and first fall frost dates are important indicator for proper timing when direct seeding, and planting tender annuals and vegetables. You can look the first and last frost dates up by ZIP code.
Hardiness Zones and Global Warming
With global warming, hardiness zones are changing. For home gardeners, this has both negative and positive consequences. Your location might become too warm for species that you planted in the past, they either might not grow as well, or be attacked by new pests that migrate further north in tandem with global warming. On the other hand, you might be able to grow plants that few years ago were not cold-hardy enough for your location.
USDA Plant Hardiness Zone Map. USDA Agricultural Research Service.