New Plant Hardiness Zone Map Reflects Warmer Conditions
The USDA released its new Plant Hardiness Zone Map (PHZM) in mid-November, 2023, 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, according to Christopher Daly, of the PRISM Climate Group at Oregon State University. This means about half of the country has moved into a warmer climate zone. This includes Albemarle County and much of Central Virginia, 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 is the national standard that most gardeners, nurseries, garden magazines, catalogs and books use to determine 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 1a being the coldest and zone 13b is the warmest.
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 shows the average lowest winter temperatures from data drawn from a span of 30 years.
The new hardiness zone map released this year was created jointly by Oregon State’s PRISM Climate Group and the USDA’s Agriculture Research Service and is described as “the most accurate and detailed it has ever released.” 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)
Based on data from local weather stations, the model included factors this time, such as topography, elevation, temperature inversions, costal effects and proximity to bodies of water, etc., to determine the impact of the hardiness zone estimations for specific locations. The new map incorporates data from 13,412 weather stations, compared to the 7,983 that were used for the 2012 edition. Viewable in a Geographic Information System-based interactive format, the map 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.
Why Is This Important?
Virginia’s Home Garden Vegetable Planting Guide: Recommended Planting Dates and Amounts to Plantuses the data from the USDA’s Plant Hardiness Zones to provide guidance on selecting appropriate planting dates for spring- and fall-planted vegetables for the home garden. 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 4/15 – 4/25. In zone 7b, the average last spring frost is basically two weeks earlier, 4/5 – 4/15. 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 types of plants, flowers, fruits and vegetables that were once unlikely to grow well in a specific location. For example, take Magnolia trees, Magnolia grandiflora. This longtime southern 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 4-7, 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 that the site is rated. Better yet, choose local native plants, appropriate for our Northern Piedmont ecoregion, 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 coevolved with our pollinators, insects and other native animals and will provide food, shelter and habitat.
In addition to taking into account the plant hardiness zone, remember there are other factors (soil fertility, quality of plant specimens and management practices) that affect plants’ ability to thrive.
In summary, 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 questions at email@example.com or 434-872-4580.