The success of growing perennial flowers, shrubs, and trees depends heavily on predicting whether the plant can survive the coldest part of the year. Gardeners need a standardized tool to match a plant’s cold tolerance to their local environment. The United States Department of Agriculture (USDA) Plant Hardiness Zone Map serves as this standard, offering a geographic guide for predicting which plants are most likely to survive winter temperatures. Understanding the zone system allows gardeners to select appropriate floral varieties, saving time and resources.
Understanding the Hardiness Zone System
The USDA Plant Hardiness Zone Map is based exclusively on the average annual extreme minimum winter temperature for a given area, not the lowest temperature ever recorded. The map divides North America into 13 primary zones, with each zone representing a 10-degree Fahrenheit temperature range.
Each primary zone is further broken down into two sub-zones, designated by ‘a’ and ‘b’, which represent a finer 5-degree Fahrenheit variation. For instance, Zone 6 is warmer than Zone 5, and Zone 7a is colder than Zone 7b. The most recent version of this map, updated in 2023, incorporates 30 years of climate data from 1991 to 2020, reflecting gradual warming trends across many regions.
The Specific Zones of New Jersey
New Jersey spans multiple hardiness zones due to significant climatic diversity. The current USDA map places the state primarily within the range of Zone 6a to Zone 7b, with a few isolated pockets reaching a warmer classification. This variation is largely due to differences in elevation, proximity to the Atlantic Ocean, and the urban heat island effect near major metropolitan areas.
The coldest areas are found in the northern part of the state, specifically the Skylands and Northwest Highlands regions. These areas, including counties like Sussex and Warren, are classified as Zone 6a or 6b. Zone 6a indicates that the average annual extreme minimum temperature falls between -10°F and -5°F, requiring the selection of highly cold-tolerant perennial flowers.
Moving into Central New Jersey, encompassing areas like Mercer and Middlesex counties, the zones transition to Zone 7a. In this zone, the lowest expected temperatures range from 0°F to 5°F. This warmer classification allows for a broader selection of plant material compared to the northern counties.
The warmest classifications are found along the Southern and Coastal regions, particularly in counties bordering the Atlantic Ocean and the Delaware Bay. Areas in Southern New Jersey, such as Cape May and Atlantic City, are predominantly classified as Zone 7b. This zone indicates average annual extreme minimum temperatures between 5°F and 10°F, offering the longest growing season and the most flexibility in perennial flower selection. A few isolated coastal sections, particularly at the very tip of Cape May, have even shifted into Zone 8a in the 2023 update.
Translating Zones into Plant Selection
The hardiness zone information serves as the primary guide for selecting flowers that will survive the winter in a New Jersey garden. When purchasing plants, gardeners should check the plant tag, which lists a recommended hardiness zone number. The plant’s listed zone must match the gardener’s zone or be a lower number to ensure winter survival. For example, a gardener in Zone 7a should choose plants rated for Zone 7, Zone 6, or lower.
While the zone map is useful, it does not account for all environmental factors that influence plant survival. These zones focus only on winter cold tolerance, neglecting the impact of summer heat, which can be damaging to certain flower species. For this reason, gardeners may also consult the American Horticultural Society (AHS) Heat Zone Map, which classifies areas based on the average number of days per year that temperatures exceed 86°F (30°C).
Localized microclimates are small areas where temperatures differ significantly from the broader zone classification. Garden areas near a building foundation, for instance, may be a half-zone warmer due to retained heat. Conversely, low-lying areas where cold air settles, or spots with poor soil drainage, can be a half-zone colder, as wet roots are more susceptible to freezing damage. Gardeners should use the USDA zone as a guide and observe specific conditions within their garden spaces.