Understanding Zones

A cold, harsh winter may have you questioning if some of your favorite plants survived. If they didn’t, it may be that they aren’t cold-hardy to our “zone.”

Back in 1960 the USDA published a Hardiness Zone Map. Updated in 2012, this map is divided into 5-degree gradients labeled numbers and further subdivided into A and B. Each zone shows the average annual extreme minimum temperature in an area.

The important word here is “average.” This map does not indicate how cold it might get in any particular winter, it shows the average lowest temperature experienced in a location.

Nearly all of Iowa is in USDA Zone 5, which has average lowest temperatures each year between -20°F and -10°F. That is air temperature; it doesn’t take into account wind chill.

While cold isn’t the only thing that determines a plant’s success in your garden, it is a key factor, especially in Iowa.

Keep in mind that you may stumble across other hardiness maps. Other organizations, such as the Arnold Arboretum in Boston and the American Horticultural Society (AHS), have developed similar maps. Additionally, the AHS has developed a Heat Zone map that shows the average number of days an area experiences over 86°F, useful in hot-weather regions of the country.

Despite these other maps, the 2012 USDA Hardiness Zone map is the new standard for gardeners, especially in the Midwest. Check it out at http://planthardiness.ars.usda.gov.

Prepared by Aaron Steil, Manager of Public Programs