The United States Department of Agriculture (USDA) Plant Hardiness Zone Map is the established standard used by gardeners and growers to predict which perennial plants can survive winter conditions in a specific geographic location. This map divides the country into distinct zones, providing a science-based guide for plant selection by identifying the coldest temperatures a plant is likely to face. Understanding the designated zone for an area like New Jersey is fundamental for successful horticulture, as it directly impacts the ability of trees, shrubs, and perennial flowers to survive the winter and return the following spring.
The Specific Hardiness Zones of New Jersey
New Jersey encompasses a range of hardiness zones, primarily spanning from Zone 6b to Zone 8a. The state’s coldest regions are found in the northwestern highlands, where the highest elevations experience minimum temperatures typical of Zone 6b. This zone indicates that the average annual coldest temperature is between -5°F and 0°F.
The central and inland southern parts of the state are predominantly classified as Zone 7a and 7b, reflecting average minimum temperatures from 0°F to 10°F. The state’s warmest designation, Zone 8a, is currently found in the extreme southern coastal area, notably around Cape May.
How Hardiness Zones Are Determined
The USDA Plant Hardiness Zone Map is created through an analysis of long-term temperature data collected from thousands of weather stations across the United States. The methodology focuses specifically on the average annual extreme minimum temperature recorded at each location. This average is calculated over a 30-year period, with the current 2023 map using data from 1991 through 2020.
Each numbered zone represents a 10-degree Fahrenheit band of minimum temperature, with the zone number increasing as the climate becomes warmer. For example, Zone 6 has an average annual minimum temperature range of -10°F to 0°F, while Zone 7 ranges from 0°F to 10°F.
To provide greater precision, each full zone is further divided into two sub-zones, designated by the letters ‘a’ and ‘b’, which represent a finer 5-degree temperature difference. The ‘a’ sub-zone is the colder half, and the ‘b’ sub-zone is the warmer half. This approach ensures the zones accurately reflect the winter survival conditions for perennial plants, though the map does not account for factors like soil type, moisture, or summer heat.
Geographic Climate Variation Within the State
The multiple hardiness zones found across New Jersey are a result of several distinct geographic and environmental factors influencing local temperatures. The higher elevations of the Skylands region in the northwestern part of the state, including parts of Sussex and Warren counties, naturally experience colder temperatures due to altitude. These cooler conditions lead to the state’s lowest hardiness zone designation, currently Zone 6b.
The expansive Atlantic Ocean acts as a warming influence, stabilizing winter temperatures along the Jersey Shore and the state’s eastern border. Water retains heat longer than land, which keeps coastal areas milder and prevents the temperature from dropping as low as inland locations, resulting in warmer zones like 7b and 8a.
The urban heat island effect also contributes to warmer zones in densely populated areas such as the major cities near the New York metropolitan area. Paved surfaces, buildings, and industrial activity absorb and re-radiate heat, raising the average annual minimum temperature relative to surrounding rural areas. This localized warming can cause a specific neighborhood to be classified as a half-zone warmer than a nearby undeveloped region. Gardeners should consider these localized microclimates when selecting plants.