While looking at plants in a garden catalog, you find one that looks perfect for your garden. The catalog states it is a Zone 2 plant; you wonder what that means and whether the plant will grow in Iowa. A plant hardiness zone map predicts the adaptability of plants to a specific climatic area. Zones are based on the average annual minimum temperatures experienced at weather stations over time. Many plants may survive in zones warmer or colder than their designated zones, but mere survival does not necessarily represent satisfactory performance. The Plant Hardiness Zone Map prepared by the Agricultural Research Service (USDA) is the most widely used in the United States. You can find the map and information about it at the following Web site: www.usna.usda.gov/Hardzone/index.html.
Iowa falls into Zones 4 and 5. Areas in the northern part of the state (Northwood) and a section in southwest Iowa have an annual minimum temperature of -25 to -20 degrees F for Zone 4b. Areas more central in Iowa (Des Moines) have an annual minimum temperature of -20 to -15 degrees F for Zone 5a. Remember, these are averages; a winter with more severe temperatures is harder on plants. When close to a boundary, choose plants hardy for the colder zone.
Microclimates and heat zones also are considerations when choosing plants, trees, and shrubs. For more information about microclimates and heat or hardiness zones, stop by or call your local ISU Extension office and ask for RG 215, Gardening in the Zones.
By Joy Rouse, Warren County Extension Education Director
10/9/2007