The USDA Plant Hardiness Zone (PHZ) map provides a geographical framework for understanding climate, particularly for agriculture and horticulture. This system divides the United States into regions based on their average annual minimum winter temperature. Ohio spans several different classifications, reflecting the state’s diverse geography and microclimates.
The Specific Hardiness Zones Found in Ohio
Ohio’s climate places the state primarily within zones 5b through 7a, according to the latest USDA Plant Hardiness Zone Map. The PHZ system uses 10-degree Fahrenheit increments for the main zone number, which is divided into “a” and “b” subzones representing a 5-degree difference. Zone designations are determined by the average coldest temperature recorded annually over a 30-year period.
The coldest areas, classified as 5b, are typically found in the far northwest and some higher-elevation inland parts of the state. These regions experience average annual minimum temperatures between -15°F and -10°F. The majority of Ohio, including central areas like Columbus, falls into the 6a or 6b subzones.
Significant variation occurs near Lake Erie, where the large body of water moderates winter temperatures. This effect, combined with urban heat islands, results in small pockets of the warmest zone, 7a, particularly along the shore near Cleveland. Zone 7a signifies an average annual minimum temperature between 0°F and 5°F, differing notably from the state’s colder interior.
Why These Zones Matter for Gardening
Understanding a location’s specific hardiness zone is essential for planning a landscape or garden. The zone designation predicts which perennial plants, trees, and shrubs can reliably survive the winter in that area. Plants are typically labeled with a zone rating indicating the coldest conditions they can tolerate.
Selecting a plant rated for a lower zone than your location, such as a Zone 5 plant for a Zone 6 area, ensures a high probability of winter survival. Conversely, choosing a plant rated for a higher zone means it may not withstand the average minimum winter temperatures and will likely fail. Local conditions, often called microclimates, can also influence plant survival within a zone.
Factors such as proximity to large buildings, paved surfaces, or a slope can create localized temperature variations that differ from the official zone map. For example, a sheltered courtyard in a major city may be slightly warmer than an open field nearby, creating a microclimate that behaves like a warmer subzone. Gardeners should use the PHZ as a primary guideline while also considering their immediate surroundings.
Other Zone Systems Applicable to Ohio
While the USDA Plant Hardiness Zone is the most common reference, other systems also apply to Ohio. One such system is the American Horticultural Society (AHS) Heat Zone map, which considers the opposite end of the temperature spectrum. The AHS system measures the average number of days each year when the temperature exceeds 86°F (30°C).
This map helps gardeners select plants that can tolerate the summer heat and humidity. Using both the USDA and AHS maps provides a more comprehensive view of a plant’s ability to survive both the winter cold and the summer heat in Ohio.
Another non-horticultural zone applicable to the state is the time zone. Ohio operates almost entirely within the Eastern Time Zone (ET), observing Eastern Standard Time (EST) in winter and Eastern Daylight Time (EDT) during the warmer months.