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The Greener View: Agricultural Hardiness Zones

Jeff Rugg on

Q: Recently, you mentioned that some ornamental grasses were only hardy to zone 5 or zone 7. Does that mean they are hardy from zones 1 through 5 or 1 through 7?

A: I am sorry for the confusion. The lower the cold hardiness zone number, the colder the average winter temperature. The lower the cold hardiness zone number, the hardier the plant is.

Cold hardiness zone maps are drawn with the coldest and lowest numbered zones starting in the north. So, zone 1 would be in northern Alaska, where the temperature reaches minus 60 F. Each zone gets 10 degrees warmer, so zone 2 is only 50 degrees below zero, and zone 3, in northern Minnesota, is minus 40 F.

The grasses mentioned as being hardy to zone 5 can tolerate minus 20 F, while the zone 7 plants can only go down to 0 F. If we start in warm climates, such as southern Florida (which is in cold hardiness zone 11, with its 40 F average winter temperature), then we may say that a plant is only hardy to zone 4 or 5.

All of the cold hardiness zone maps use the average annual minimum temperature, which is not the same as the coldest minimum temperature that occurred each year during that time. For example, zone 6, which averages 10 degrees below zero, may have had several winters during the 10 to 30 years of data collection during which the temperature went to minus 21 or minus 27 and several years where the coldest temperature only went to minus 4 degrees. But the overall average is still 10 degrees below zero. Planting trees that you want to live more than a few years will require looking at more information than just the hardiness zone map and its averages.

So how should a hardiness zone map be used? Gardeners need to keep in mind that all cold hardiness zone maps only account for one factor in a plant's environmental needs for growth and survival. All of the following factors can influence a plants survival: heat; street lights versus day length; soil toxins; airborne toxins; acid rain; fertilizer; watering; pest control; location (a yard versus sidewalk planter box); microclimates; spring and fall frosts; soil pH; soil aeration; and many others.

Even though Seattle, Washington, Dallas, Texas, and Tallahassee, Florida, are all in cold hardiness zone 8, they have very different climates when you consider all the other weather factors.

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Plus, every individual landscape has small microclimatic differences. For instance, the flower beds in front of a south-facing brick wall may be a cold hardiness zone or two warmer than the flower beds in the shady north side of the same building, because they never get as cold.

Unfortunately, there are no maps telling gardeners if they live in an area that is too wet, dry, sunny or shady or any other information aside from average temperatures. The heat zone maps that have been produced in the past are even less reliable than the cold hardiness maps and are only useful in a general way.

If you want to plant long-lifespan trees, shrubs and perennials such as peonies, then consider that the cold hardiness map is only recording average lows and not extreme lows. You may want to get plants hardy to at least one zone colder than the map or plant catalog indicates, just so it can survive record-cold winter temperatures. Of course, you will still want to consider all of the other aspects of the climate, soil, sunshine and watering that the plant will need to survive as you make your final decision.

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Email questions to Jeff Rugg at info@greenerview.com. To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

 

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