The survival of perennial plants depends heavily on the coldest temperatures they must endure through the winter. Gardeners and agricultural professionals rely on the United States Department of Agriculture (USDA) Plant Hardiness Zone (PHZ) map to determine which trees, shrubs, and flowers can withstand the annual winter cold in a specific area. This standardized measure guides successful gardening and agricultural planning. Understanding this system and the zones designated for Arkansas is the first step toward selecting a landscape that will thrive.
Defining the USDA Plant Hardiness Zone System
The USDA Plant Hardiness Zone map is the accepted standard for measuring a region’s winter cold tolerance. The entire system is structured around the average annual minimum winter temperature, which is the single coldest temperature expected in a year, averaged over a long period. The map divides North America into 13 primary zones, with each full zone representing a 10-degree Fahrenheit temperature band.
For greater precision, each of the full zones is further divided into two subzones, designated ‘a’ and ‘b’. These subzones represent a 5-degree Fahrenheit difference in the average annual minimum temperature. The ‘a’ subzone is the colder half of the full zone, while the ‘b’ subzone is the warmer half.
The USDA periodically updates the map to ensure the data remains current with long-term weather patterns. For instance, the 2023 revision utilized three decades of temperature data, from 1991 to 2020, to reflect changes in climate. This update showed a general warming trend across the country, shifting many areas, including parts of Arkansas, into a slightly warmer half-zone.
Arkansas’s Designated Hardiness Zones
Following the 2023 revision of the USDA Plant Hardiness Zone map, Arkansas now spans a range from Zone 7a to Zone 8b. This means that the state’s average annual minimum winter temperatures range from 0 degrees Fahrenheit to 20 degrees Fahrenheit. The previous map had included areas of the colder Zone 6, but the warming trend has shifted nearly all of the state into the warmer Zones 7 and 8.
The distribution of these zones across Arkansas is directly tied to the state’s geography and elevation. The northern and mountainous regions, particularly the Ozark Highlands, fall into the cooler part of the state, primarily designated as Zones 7a and 7b. Cities like Fayetteville and Bentonville experience these slightly cooler winter minimums. Zone 7a ranges from 0 to 5 degrees Fahrenheit, while Zone 7b is slightly warmer, ranging from 5 to 10 degrees Fahrenheit.
The central, eastern, and southern portions of Arkansas generally fall into the milder Zones 7b, 8a, and 8b. The Arkansas River Valley and the Delta region see warmer winter minimums. Major population centers, including Little Rock, Fort Smith, and Conway, are typically situated in Zone 8a. This zone, which is the dominant hardiness zone in the state, has a minimum temperature range of 10 to 15 degrees Fahrenheit.
The warmest parts of the state, particularly the far southern and southwestern counties, are classified as Zone 8b. This zone represents the highest winter temperature average in Arkansas, with minimum temperatures falling between 15 and 20 degrees Fahrenheit. The range of zones highlights the need for gardeners to look up their specific location rather than relying on a single zone for the entire state.
Using Zone Data for Successful Planting
The plant hardiness zone number allows gardeners to select perennial plants that can survive the coldest expected winter temperatures in their specific area. When purchasing perennials, trees, or shrubs, gardeners should look for the corresponding USDA zone number listed on the plant tag or seed packet. If a plant is labeled as “hardy to Zone 7,” it is expected to survive the minimum temperatures experienced in Zone 7 and all warmer zones.
The hardiness zone is only a guideline based on a long-term average and not a guarantee. Gardeners should factor in local variations, known as microclimates, which can slightly alter the effective zone of a small area. For instance, a protected courtyard or a south-facing wall may retain more heat, creating a warmer pocket than the map indicates. Conversely, a location on a high ridge or in a low-lying valley where cold air settles may be slightly colder than the designated zone.
While the hardiness zone addresses winter survival, it does not account for other factors that influence a plant’s health, such as summer heat, rainfall, or soil quality. A plant’s ability to thrive depends on a balance of these elements, not just cold tolerance. The zone map is foundational information that must be used alongside knowledge of a plant’s other requirements and the specific conditions of the property.