The Plant Hardiness Zone (PHZ) system, maintained by the U.S. Department of Agriculture (USDA), classifies geographic areas based on temperature data for horticultural purposes. The zones reflect the lowest winter temperatures a perennial plant is likely to encounter. New Jersey encompasses a range of these zones, meaning gardeners must understand their specific local zone to select plants that will survive the winter. A single zone designation is impossible due to this variation across the state.
Defining the USDA Plant Hardiness System
The USDA system is based on the average annual extreme minimum winter temperature, calculated from the coldest temperature recorded over a 30-year period. This metric defines each zone, reflecting typical winter survival conditions for perennial plants.
The map divides the country into 13 zones, with each full zone representing a \(10^\circ \text{F}\) temperature band. For greater precision, each zone is split into two subzones, ‘a’ and ‘b,’ representing a \(5^\circ \text{F}\) difference. Zone 6a is colder than Zone 6b, providing a detailed metric for matching a plant’s cold tolerance to a specific location. This metric assesses cold-weather survival, not a plant’s ability to thrive during the summer or other seasons.
New Jersey’s Specific Hardiness Zones and Ranges
New Jersey’s hardiness zones span a range, generally running from Zone 6b in the coldest northwest areas to Zone 8a at the warmest southern tip of the state. The coolest region is the Northwest Highlands and Skylands, which primarily fall into Zone 6b. This zone indicates average annual extreme minimum temperatures are between \(-5^\circ \text{F}\) and \(0^\circ \text{F}\).
Much of the central and southern inland portions of the state, including the Pine Barrens, are classified as Zone 7a or 7b. Zone 7a indicates minimum temperatures typically range from \(0^\circ \text{F}\) to \(5^\circ \text{F}\), while 7b ranges from \(5^\circ \text{F}\) to \(10^\circ \text{F}\). The geographic variation is influenced by two major factors: the moderating effect of the Atlantic Ocean and the presence of urban areas.
The coastal strip and barrier islands often sit in the warmer Zone 7b due to the ocean’s thermal mass moderating winter temperatures. Additionally, the dense urban corridor experiences the urban heat island effect. This phenomenon causes metropolitan areas to retain heat, pushing their zones into warmer classifications compared to surrounding rural areas. The warmest microclimate in the state is found at the extreme southern coast of Cape May, which has been designated as Zone 8a, reflecting minimum temperatures between \(10^\circ \text{F}\) and \(15^\circ \text{F}\).
Applying Zone Data to Plant Selection
Knowing the local hardiness zone provides a critical step for gardeners when selecting perennial plants, trees, and shrubs. The zone number listed on a plant tag indicates the coldest zone where that specific plant is expected to survive the winter. Gardeners should choose plants rated for their local zone or any lower (colder) zone to ensure winter survival. For instance, a gardener in Zone 7a can reliably plant species rated for Zone 7, 6, or 5, but planting anything rated for Zone 8 carries a significant risk of winter death.
The PHZ system has limitations, as it exclusively measures cold tolerance and does not account for other factors that influence a plant’s success. The map offers no information about summer heat, rainfall, soil quality, or light requirements. Gardeners should also consider microclimates, which are small areas within a property that are slightly warmer or cooler than the general zone. This detailed environmental awareness, combined with the zone data, enables a more successful and sustainable landscape plan.