Cedar Valley Arboretum & Botanic Gardens

Do you know Black Hawk County’s Plant Hardiness Zone?

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You have heard me praise the Arboretum’s e-newsletter in the past.  Our most recent edition – read full here http://www.cedarvalleyarboretum.org/pdfs/The%20View%20Volume%204%20Issue%201.pdf – is no exception.  Co-editor Pat McGivern wrote an incredibly informative article about plant hardiness zones and I wanted to share with you.  I know you will enjoy it as much as I did!

Thank you, Pat!

Would you like to receive the Arboretum’s monthly e-newsletter, The View?  Sign up here http://www.cedarvalleyarboretum.org/newsletter.asp.


Winter is the season we gardeners speculate and dream about our gardens for the coming year. In December and January we are deluged with bright and arty garden catalog mailings, that tempt us to try new plant varieties. But do we know what is truly expected to survive in our backyard through the winter a year from now?

Most gardeners have seen the United States Department of Agriculture (USDA) Plant Hardiness Zone Map (above) that is used as a standard guide to a plant’s cold tolerance. This map divides the United States and southern Canada into 11 zones, defined by a 10 degree Fahrenheit difference in the average annual minimum temperature, then subdivided by a 5 degree difference. The higher the zone number, the warmer the temperature. The original USDA map was published in 1960, sponsored by the United States National Arboretum and the American Horticultural Society (AHS), and was last officially revised in 1990, using temperature data gathered from 1974 to 1986. Using that map, Black Hawk County is clearly in Zone 4b, with average minimum temperatures of -20 to -25 degrees Fahrenheit.

In 2003, the AHS produced a draft revised map, using temperature data gathered from July 1986 to March 2002, which was a period of warmer winters than the last 1990 USDA revised map. That map would place Black Hawk County in the warmer Zone 5, with average minimum temperatures of -20 to -10 degrees Fahrenheit. However, this draft was not accepted by the USDA. As of January 2011, both the USDA and the National Arboretum websites are using the 1990 USDA map as current. The USDA website states that the USDA “is in the process of creating another version of the hardiness map using new mapping technology and an extended set of meteorological data.”

In 2006, the National Arbor Day Foundation used weather data from 1991 to 2005 to create their own plant hardiness zone map. The Arbor Day Foundation map also places Black Hawk County in Zone 5, with average winter minimums of -20 to -10 degrees Fahrenheit.

Are you confused yet? It remains the standard practice of most seed dealers and nurseries to label their products by the USDA plant hardiness zones (Black Hawk is then Zone 4b). You should be safe in planting perennials that are labeled for Zone 4, or for Zone 3 which is north of us. However, you can consider whether you wish to test the hardiness zone issue with trying a Zone 5 plant. Also consider the microclimate of where the plant will be placed. Will it be in a sunny location, sheltered from the harshest winds? Will it be in an area that usually has plenty of snow cover, to insulate the roots? Will you plan to mulch or cover the Zone 5 plant which may or may not be a bit tender for our next winter?

Good luck! As noted by author Michael Pollen in Second Nature: “We gardeners have always had trouble heeding Henry Ward Beecher’s sound nineteenth century advice, that we not be ‘made wild by pompous catalogs from florists and seedsmen.’ “

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Written by cedarvalleyarboretum

January 27, 2011 at 6:00 am

Posted in Uncategorized

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