From the Ground Up: So what’s my growing zone?

If you're new to gardening you'll need to familiarize yourself with growing zones and what they mean. If you are a seasoned gardener, you still might be confused about zones. The U.S. Department of Agriculture developed a Plant Hardiness Zone Map which is the standard guide to a plant's cold tolerance. The map is based on the average annual minimum winter temperature, divided into 10-degree F incremental zones. The U.S. is divided into 13 zones based on average annual minimum temperatures. Zone 1 is the coldest with temps dropping to minus 60. Zone 13 is tropical with temps not going below 60. The USDA has been keeping this data for over 40 years.

Most of Iowa is in Zone 5, with some areas in the northern part of the state listed as Zone 4. But the USDA Zone map breaks it down further with Zone 4b (minus 25 to minus 20), Zone 5a (minus 20 to minus 15), Zone 5b (minus 15 to minus 10) and so on. The USDA map is available as an interactive GIS-based map, at www.usna.usda.gov/Hardzone/. This map also allows you to plug in your zip code to find out your exact planting Zone. My zip code in Linn County places me in Zone 5a, but I have lost Zone 5 hardy plants in my garden. The zone map is important when choosing trees, shrubs and perennials for your yard. Almost all nurseries, plant catalogs and garden magazines use the zone to describe plants.

In 1997, the American Horticulture Society developed yet another map tool for gardeners, the Plant Heat Zone Map, much less referenced than the USDA cold hardy zone map, but still helpful. This map is a guideline for a plant's heat tolerance. This map is particularly helpful in choosing turf grass species. The AHS heat map is based on the average number of days in the year when temps reach or exceed 86 degrees. The AHS choose 86 degrees because that's the point when plants can suffer physiological damage from heat. Similar to the USDA map, the AHS Heat map ranges from Zone 1 to 12, with Iowa ranging from Zones 4 in the northeast part of the state, to Zone 7 in the extreme southwest corner.

These maps are intended to be a guide to help you select plants that will be most successful for your area. When the weather doesn't cooperate and we see extremes such as the dangerously low temps a few winters back, plant damage and death can certainly occur. You can grow more tender plants but you may lose them, if you don't plan on bringing them in for the winter.

Please plan to attend the Linn County MG Winter Gardening Fair. Register online at: www.extension.iastate.edu/linn/content/winter-gardening-fair.

l For gardening questions, call the Linn County Extension Master Gardener Hotline at (319) 447-0647.