Outdoor gardening success relies heavily on understanding a location’s climate, particularly the severity of winter. Plants must withstand the coldest annual temperatures to survive and return season after season. This understanding is standardized through a geographical zoning system used by horticulturalists. Identifying this zone is the foundational step for any gardener to determine which perennial species can thrive without protection or replacement.
Understanding the USDA Hardiness Zone System
The standard reference for assessing a region’s climate suitability for perennial plants is the United States Department of Agriculture (USDA) Plant Hardiness Zone Map. This system is based entirely on one climatic factor: the average annual minimum winter temperature. It does not factor in other variables like rainfall, soil type, or maximum summer temperatures.
The map is divided into zones, each representing a 10-degree Fahrenheit range of minimum winter cold. To provide greater precision, each zone is segmented into ‘a’ and ‘b’ subdivisions, representing a narrower 5-degree Fahrenheit temperature difference. For example, a plant rated for Zone 6 will survive colder temperatures than a plant rated for Zone 7. This designation indicates the lowest temperature a plant can tolerate for long-term survival in that area.
New Jersey’s Specific Hardiness Zone Designations
New Jersey is geographically diverse, spanning two primary hardiness zone groups: Zones 6 and 7. This variation means gardeners must consult their local zone rather than relying on a single state-wide number. The state exhibits a gradient, moving from colder zones in the interior and northern regions to milder zones along the coast and in the south.
The northern and inland portions of New Jersey, including counties like Sussex, Warren, and parts of Morris, are designated as Zone 6a and 6b. These areas experience the coldest minimum temperatures, averaging between -10 and 0 degrees Fahrenheit. This reflects the continental climate influence and higher elevations found in the state’s northern reaches.
Moving south toward the Atlantic coastline, the zones become progressively warmer due to the ocean’s moderating effect. Central and Southern New Jersey, encompassing areas like Atlantic, Cape May, and parts of Ocean County, are classified as Zone 7a and 7b. The average annual minimum winter temperatures here range from 0 to 10 degrees Fahrenheit. The Atlantic Ocean prevents temperatures from plummeting as severely as they do in the northern, inland areas.
Applying Zone Knowledge to Planting Decisions
Knowing the specific hardiness zone is the most direct way to ensure the long-term survival of perennial plants. Gardeners should always check the label of a perennial plant, shrub, or tree before purchasing; it lists the lowest USDA zone in which the plant can reliably survive. Plants rated for the gardener’s zone or a lower number (meaning they tolerate colder temperatures) are safe choices.
If a plant is rated for a higher zone (e.g., planting a Zone 8 plant in Zone 6), it will likely not survive the average winter without extensive protection. While the USDA zone provides a strong baseline, gardeners must also consider small-scale variations known as microclimates within their property. Areas near a large, south-facing brick wall or a body of water can be slightly warmer than the surrounding landscape, sometimes allowing the successful growth of plants rated for a slightly higher zone.