Is Your Cold Hardiness Map Useful? by Jeff Rugg on Creators.com

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Is Your Cold Hardiness Map Useful?

Last week, we looked at the new U.S. Department of Agriculture hardiness zone map and the fact that it does not prove global warming. Using 30 years of one measure of weather to create a map for gardeners while ignoring all of the other weather variables that climatologists use may excite the blogging world, but it doesn't prove much to real scientists. To indicate true change, a climate map would need to show data from all the measurable weather aspects for 50 to 100 years just to begin to provide reliable data for climatologists.

In the new USDA map, some towns across the country moved into new 5-degree zones, both warmer and colder, by having their 30-year average coldest-winter temperature shift less than a single degree. Both Tampa, Fla., and Cheyenne, Wyo., moved up 5 degrees on the map, while Pierre, S.D., moved down 5 degrees, all with less than half a degree of average real temperature change. Small real differences within the margin of error of reading a thermometer created big map differences.

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, a zone that averages minus 10 degrees may have had several winters during the 10 to 30 years of data collection where the temperature went to minus 21 or minus 27 and several years where the coldest temperature only went to minus 4 degrees. Altogether, over 30 years of record cold and record not-so-cold temperatures, the average is minus 10.2 degrees. Planting trees that you want to live more than a few years will require looking at more information than an "average"-based hardiness zone map.

How should a hardiness zone map be used? Gardeners need to keep in mind that the map accounts for only 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 and airborne toxins; acid rain; fertilizer; watering; pest control; location (a yard versus a sidewalk planter box, for instance); microclimates; spring and fall frosts; soil pH; soil aeration; etc.

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

Every individual landscape has small microclimatic differences. The flowerbeds in front of a south-facing brick wall may be a cold hardiness zone or two warmer than the flowerbeds on the shady north side of the same building because they never get as cold.

Many gardeners want to grow plants that aren't really capable of growing in their landscape for a variety of reasons. For certain plants, the landscape may be too wet or dry; too sunny or shady; or be located in a region that gets too warm in the summer or too cold in the winter.

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 other than average cold and average heat. 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.

Every year, growers produce new varieties of plants that are hardier than the older varieties. Seeing a new, hardier plant from an old southern species may make some people think they're seeing evidence of a warming trend when it's just an improved plant growing in a colder location than usual.

You can use the current hardiness maps for recommendations of what will survive if you want to grow short-lived perennials. If they don't survive, you can plant more. You can look for the hardiest varieties, and you can move them or cover them in winters with record lows instead of average low temperatures.

If you want to plant long life-span trees, shrubs and even some perennials such as peonies, then consider that the cold hardiness map is recording only average lows and not extreme lows. You may want to get plants hardy to at least one zone colder than what's indicated by the map or plant catalog to ensure that it survives the record-cold winter temperatures. Of course, you'll 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.

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.