The USDA Plant Hardiness Zone Map was updated in November 2023 to provide a more accurate and detailed guide for gardeners and growers. The changes reflect more recent temperature data and improved mapping technology.
How the Zones Work
The USDA map is the standard for determining which perennial plants are most likely to survive the winter in a given location.
- Basis: The zones are based exclusively on the average annual extreme minimum winter temperature, which is the single coldest night of the year, averaged over a 30-year period.
- Zones and Subzones: The country is divided into 13 major zones, each representing a 10°F temperature range. Each major zone is further divided into two 5°F half-zones, labeled “a” and “b” (e.g., Zone 4a is colder than Zone 4b, and 4b is colder than 5a).
- Purpose: If a plant is rated as hardy for your zone, it means it can typically survive an average winter minimum temperature in that area.
Understanding the 2023 USDA Plant Hardiness Zone Map …
Mar 23, 2024 — Basil & Salt Magazine delves into the significance of these updates, shedding light on the modifications and the impli…
How and Why the Map Changed
The 2023 map, developed by the USDA Agricultural Research Service (ARS) and the PRISM Climate Group at Oregon State University, incorporated several key updates:
- New Data Period: The map uses temperature data from 1991 to 2020, compared to the 1976-2005 data used for the 2012 map.
- More Weather Stations: The new map incorporates data from over 13,000 weather stations, nearly double the number used for the 2012 map. This provides much greater detail and accuracy.
- Improved Mapping Methods: More sophisticated algorithms were used to estimate temperatures in areas without direct weather station data (e.g., mountains, areas near large bodies of water), leading to finer detail and more localized zones.
- Overall Shift: On average, the 2023 map is about one-quarter zone warmer across the contiguous United States compared to the 2012 map. This meant about half of the country shifted to a warmer half-zone.
While climate change is a significant factor in general warming trends, the map developers caution that the changes in hardiness zones alone are not a direct indicator of global climate change, as they focus specifically on the extreme minimum temperatures, which can be highly variable year to year.
Changes in the Black Hills, South Dakota
Much of South Dakota experienced a shift to a warmer zone or subzone. Areas previously in Zone 4 often shifted to Zone 5.
- The Black Hills Region: The Black Hills area has historically been a warmer pocket (often Zone 5) compared to the surrounding areas of South Dakota (which were predominantly Zone 4).
- Why it’s warmer: The topography of the Black Hills creates unique microclimates. During winter, warm air can overlie cold air that settles into low-elevation valleys, resulting in warmer temperatures in some specific areas within the Hills.
- The 2023 Change: While some parts of the Hills may have simply moved from one subzone to another (e.g., 5a to 5b), the overall trend is a reinforcement of the area’s relative warmth compared to the rest of the state.
You can use the official USDA interactive map to enter your specific ZIP code and find your exact 2023 hardiness zone.
Knowing your specific zone will help you confirm which of the plants on your ‘want’ list are best suited for your exact location.