Gardeners rely on specific climate data to determine which plants can survive in their location. This information is standardized across the United States by the United States Department of Agriculture (USDA) Plant Hardiness Zone (PHZ) map. This map establishes a geographical framework to predict the likelihood of perennial plants surviving the coldest part of the year. Understanding the growing zone for a specific area, like Delaware, is the first step in making informed choices about trees, shrubs, and other long-lived landscape additions. The zones are based solely on the extreme cold temperatures a plant must endure.
How the USDA Plant Hardiness Zone System Works
The methodology behind the PHZ map focuses entirely on one specific climatic measurement: the average annual extreme minimum winter temperature. This figure is calculated using weather data collected over a 30-year period to determine the lowest temperature a location is expected to reach during an average winter. The entire system is divided into 13 main zones, with each one representing a 10-degree Fahrenheit difference in this average minimum temperature.
Each of these main zones is further split into two subzones, labeled ‘a’ and ‘b,’ to provide a more granular level of detail. Subzone ‘a’ represents the colder half of the 10-degree range, while ‘b’ represents the warmer half, creating a 5-degree Fahrenheit distinction between them. For instance, Zone 6 covers a 10°F span, but 6a is 5°F colder than 6b, allowing growers to select plants with greater precision.
The Specific Zones of Delaware
Delaware’s small geographical area contains a range of growing environments that place it primarily within USDA Hardiness Zones 7a, 7b, and a few small pockets of 8a. The majority of the state falls squarely within Zone 7b, indicating a climate where the average annual extreme minimum winter temperature ranges between 5°F and 10°F. This classification reflects a generally temperate climate that is moderated by the state’s proximity to the Atlantic Ocean and the Delaware Bay.
The northernmost part of the state, particularly the inland areas of New Castle County near the Pennsylvania border, falls into the cooler Zone 7a. This subzone has an average annual minimum winter temperature range of 0°F to 5°F, which can necessitate selecting plants with greater cold tolerance. As one moves south through Kent County and into the non-coastal areas of Sussex County, the designation shifts to the warmer 7b, which covers the central plain of the state.
The influence of the ocean becomes most apparent along the southern coast and beach communities, where the climate is further moderated, creating small microclimates designated as Zone 8a. This classification indicates that these areas experience average minimum winter temperatures between 10°F and 15°F, allowing for the successful growth of some marginally hardy plants. The most recent 2023 update to the USDA map, which used temperature data from 1991 to 2020, confirmed a warming trend by shifting many areas of Delaware a half-zone warmer compared to the previous 2012 map.
Applying Zone Information to Your Garden
The primary purpose of knowing Delaware’s hardiness zone is to match a plant’s documented cold tolerance to the local climate, ensuring its survival through the winter. When selecting trees, shrubs, or perennial flowers, gardeners should look for the PHZ rating on the plant tag and choose those rated for their local zone or a lower, colder zone. A plant rated for Zone 6, for example, will reliably survive the winter in any Zone 7 location, providing a margin of safety.
The PHZ map only addresses cold tolerance and does not account for other environmental variables that affect plant health. Factors like summer heat, rainfall, and specific soil conditions are not measured by this system. Delaware gardeners must also consider localized factors such as salt spray near coastal areas or the heavy clay soils found in the northern Piedmont region.
The zone designation also does not predict the timing of the last spring frost, which is a separate factor dictating when tender vegetables and annual flowers can be safely planted. While the PHZ informs on perennial survival, gardeners must consult local frost date tables to determine the proper time for seasonal planting. Utilizing the hardiness zone alongside an understanding of these other microclimatic factors allows for successful gardening outcomes.