The United States Department of Agriculture (USDA) evaluated the growing conditions across the United States and Canada and developed an algorithm that generated 11 agricultural zones based upon average low temperatures. USDA zones 2 through 10 are further divided into sections "a" and "b." Ohio, with its winter temperatures of 0 to -20 degrees Fahrenheit, lays in zones 5 and 6.
The USDA hardiness zone 5a is present in only a few small areas of Ohio. The winter temperatures can drop as low as 15 to 20 degrees below zero Fahrenheit. The plants grown in zone 5a must be winter hardy. Your winter temperatures may not be as cold, or you may provide protection for the plants during the winter, which offers you a larger selection of plants to grow in your area.
Most of northern Ohio has annual minimum temperatures of -10 to -15 degrees F and is classified as USDA hardiness zone 5b. The average last frost of the season ranges from March 30 to April 30. The first frost of the season generally comes in October. Plants needing a long growing season, such as watermelon, tomatoes and winter squash, should be started indoors six to eight weeks before the last expected frost date.
USDA hardiness zone 6a covers much of the southern part of Ohio. The winter temperatures can drop to -10 degrees below zero F. Plants sold in nurseries and greenhouses are generally marked with growing requirements and the hardiness zone. A plant marked as hardy from zones 6 through 9 can withstand the lower temperatures of southern Ohio winters but may not do well in the northern part of the state.
Along the Ohio river, which borders southern Ohio, there is a small section which is classified as USDA hardiness zone 6b. The winter temperatures may fall to -5 below zero F during the coldest part of the year. Because of the slightly warmer temperatures, the growing season may be longer in hardiness zone 6b. Fruits and vegetables have a longer time to ripen. As with any planting zone in Ohio, you must determine the microclimate of your garden area before planting.
The hardiness zones are a guideline for gardeners to use when planning a garden. However, the elements and other outside factors may create a bubble around your growing area. These bubbles are called microclimates. High winds or excessive rainfall may lower the annual average temperature for your growing area. This means that you may live in hardiness zone 6a but have temperatures more in line with zone 5b. Large cities often have higher temperatures than outlying suburbs and therefore the microclimate may be different than what is on the map. To determine your planting zone, track the annual low temperatures. Match the temperature records with the USDA hardiness zone map for planting a successful garden.
Julie Richards is a freelance writer from Ohio. She has been writing poetry and short stories for over 30 years, and published a variety of e-books and articles on gardening, small business and farming. She is currently enrolled at Kent State University completing her bachelor's degree in English.