Determining the correct hardiness zone is the first step toward successful gardening in Rhode Island. This classification system provides a standardized, geographically specific guide for selecting plants that can survive the region’s winter temperatures. Knowing the local hardiness zone prevents the disappointment of losing perennial plantings to an unexpected cold snap.
Defining the USDA Hardiness Zone System
The USDA Plant Hardiness Zone (PHZ) map is the standard reference tool for assessing a plant’s ability to survive winter temperatures. The map divides the United States into thirteen zones based on long-term weather data. Zones are calculated using the average annual minimum winter temperature over a 30-year period, which is the single metric the system measures.
Each major zone represents a 10-degree Fahrenheit band of minimum temperature. For greater precision, each zone is broken down into two sub-zones, labeled “a” and “b,” representing a narrower 5-degree Fahrenheit range. For example, Zone 7 means the average annual extreme minimum temperature falls between 0°F and 10°F. The “a” designation is the colder half of the range, while “b” is the warmer half.
Rhode Island’s Specific Hardiness Zones
Rhode Island, a relatively small state, encompasses a narrow but significant range of hardiness zones, primarily falling into Zones 6b, 7a, and 7b. The 2023 update shifted much of Rhode Island into a half-zone warmer classification compared to the previous 2012 map. This general warming trend means the extreme minimum temperatures are now slightly higher.
The state’s geography heavily influences these zone variations. Cooler, inland northwest towns, such as Foster and Glocester, fall into Zone 6b, where the average minimum winter temperature is between -5°F and 0°F. These areas are less influenced by the Atlantic Ocean’s moderating effects. Moving toward the coast, the zones increase in number, indicating warmer conditions.
The central and eastern parts of the state, including Providence and the Narragansett Bay area, typically fall into Zone 7a (0°F to 5°F). Providence also benefits from an urban heat island effect, contributing to this warmer zone classification. The warmest locations, such as Block Island and parts of Newport County immediately along the shore, are classified as Zone 7b. These coastal and island areas are shielded from severe cold by the large thermal mass of the ocean.
Utilizing Zone Data for Plant Selection
The practical application of hardiness zone data is selecting appropriate perennial plants, trees, and shrubs that can survive the lowest expected winter temperatures. Plants sold in nurseries are labeled with a hardiness rating indicating the coldest zone they are expected to survive. A plant rated for Zone 6, for instance, should tolerate an average annual minimum temperature down to -10°F.
Gardeners in a Zone 7a location should choose plants rated for Zone 7 or a lower-numbered zone, such as Zone 6 or 5. Selecting a plant rated for a colder zone provides a margin of safety against unusually harsh winters. The risk increases when attempting to cultivate a plant rated for a higher, warmer zone, such as a Zone 8 perennial, which is not adapted to survive Rhode Island’s average annual minimum temperatures.
This selection process is primarily concerned with the plant’s ability to overwinter. Choosing the correct zone ensures the perennial’s root system and dormant buds survive the coldest part of the year. Aligning the plant’s cold tolerance with the local zone is a foundational decision for long-term success.
Accounting for Local Growing Conditions
While the USDA Hardiness Zone map is an excellent starting point, it is only one variable in determining plant success. Small-scale variations in a property’s environment, known as microclimates, can significantly affect how a plant fares. For instance, a plant situated on the south side of a brick wall or near a large body of water will experience warmer conditions than the official zone rating suggests.
The physical characteristics of the soil are also a major consideration in Rhode Island. Most soils are derived from glacial till, resulting in a generally sandy and acidic composition. The sandy texture provides good drainage in many areas, but it also means the soil may have a lower capacity to retain water and nutrients. Gardeners must amend the soil to increase organic matter and adjust the pH to suit the needs of specific plants.
Another factor ignored by the hardiness zone map is summer heat, which can cause physiological damage to intolerant plants. The American Horticultural Society (AHS) Heat Zone map addresses this by classifying areas based on the average number of days the temperature exceeds 86°F. Using both the PHZ and AHS maps provides a more complete picture of the temperature extremes a plant will face throughout the entire year.