By Fern Campbell
Piedmont Master Gardener
The USDA released its new Plant Hardiness Zone Map this past fall, updated from the 2012 map. The big news is that the 2023 map generally shows temperatures about 2.5 degrees Fahrenheit warmer than the 2012 map across the United States. This means about half of the country has moved into a warmer hardiness zone. Here at home, Albemarle County and much of Central Virginia are now classified as zone 7b, rather than the previous 7a. Elsewhere in the Commonwealth, nearly every city in the Hampton Roads region has moved from zone 8a to zone 8b.
The USDA map—accessible at planthardiness.ars.usda.gov—is the national standard for determining the perennial plants most likely to survive the coldest winter temperatures at a certain location. This map divides North America into 13 separate zones. Each zone is 10 degrees F. warmer (or colder) in an average winter than the adjacent zone. Many of the zones are further divided into “a” and “b” regions, zone a being the colder and zone b the warmer.
Hardiness refers to a plant’s ability to withstand low winter temperatures and thrive. These hardiness zones do not represent the coldest it will ever be in an area but rather show the average lowest winter temperatures from data drawn from a span of 30 years.
The new hardiness zone map was created jointly by Oregon State University’s PRISM Climate Group and the USDA’s Agriculture Research Service and is described as the most accurate and detailed ever released by the agency. USDA used an algorithm derived from the PRISM climate mapping model to estimate the mean annual extreme minimum temperature for each pixel on the map. (PRISM stands for Parameter-elevation Regressions on Independent Slopes Model.)
With data from local weather stations (including topography, elevation, temperature inversions, coastal effects and proximity to bodies of water), the model was used to determine the impact of the hardiness zone estimations for specific locations. Viewable in a Geographic Information System-based interactive format, the new map incorporates data from 13,412 weather stations, as compared with the 7,983 used for the 2012 edition. It is based on the 30-year averages from 1991 to 2020. The 2012 edition was based on averages from 1976 to 2005.
Features of the Updated Map
Users can find the hardiness zone for their specific area using a ZIP Code zone finder tool. The new map is also accompanied by an expanded “Tips for Growers” feature with much valuable information and additional resources.
Why Is This Important?
Virginia’s Home Garden Vegetable Planting Guide (Virginia Cooperative Extension publication 426-331) uses the data from the USDA’s Plant Hardiness Zones to provide guidance on selecting appropriate planting dates for spring- and fall-planted vegetables. Each hardiness zone has an average last spring frost and first killing fall frost date.
For example, in zone 7a, the average last spring frost date is April 15–April 25. In zone 7b, the average last spring frost is basically two weeks earlier, April 5–April 15. However, these maps are only general guides. As gardeners know well, a killing frost can occur much later than the “average,” and it is important to continue to monitor your local weather predictions before planting those tomatoes.
In some areas, the new hardiness zone means we can now grow flowers, fruits, vegetables and other plants that were once unlikely to fare well in a specific location. Take, for instance, the Southern Magnolia (Magnolia grandiflora). Long associated with the Deep South, this tree could now have a chance to thrive in parts of New England due to the shift. On the other hand, those Fraser fir trees (Abies fraseri) that do best in zones 4 to 7 will be stressed in zone 7b due to their lack of heat tolerance and other heat-related challenges.
Some plants, such as apples, need long periods of cold in which the plants go dormant.
If they are not exposed to a certain number of days with minimum temperatures, they will not flower and set fruit. There is a reason why Florida is not known as the apple state.
Each woody landscape species has a designated hardiness rating. For example, the Eastern Redbud (Cercis canadensis) has a plant hardiness rating of zone 4 to 9. The number 4 refers to the lowest temperatures it can tolerate, and the 9 refers to the highest temperature this plant can tolerate and be able to grow successfully. We can expect that most plants that have grown in the North will have the genes to tolerate the zone 4 winters, and plants from the South will have genes to tolerate the higher summer heat temperatures. Thus, when purchasing plants, the geographical source of the plant, known as the provenance, may be an important consideration.
Even though some plant hardiness zones have shifted, gardeners don’t have to go out and buy all new plants; most of what we are used to growing will still thrive. Just be conservative when trying new plants. For example, consider buying a species rated one-half or even one full zone colder than the site is rated. Better yet, choose native plants appropriate for our Northern Piedmont region that, when properly sited, are adapted to local conditions. These plants will contribute to the biodiversity of the landscape and, once established, require little extra water, fertilizer or pesticides. They have co-evolved with our pollinators, insects and other native animals and will provide them with food, shelter and habitat. (For a list of Northern Piedmont natives, visit piedmontmastergardeners.org and look under the Resources tab.)
In addition to taking into account the plant hardiness zone, remember there are other factors that affect plants’ ability to succeed, including soil fertility, quality of plant specimens and management practices.
So, buy the best quality plant available for the appropriate site and hardiness zone, follow good planting practices, and water well during the establishment period. Scout for problems and resolve them promptly. And don’t forget to reach out to the local Virginia Cooperative Extension Helpdesk for advice at [email protected] or (434) 872-4583.