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Helenruth Schuette/Beltrami Master Gardener: The FYI on using the new hardiness zone map

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Using the new hardiness zone map (HZM) is only one aspect of choosing perennials for your garden.

A hardiness zone map provides information to help gardeners determine which herbaceous and woody perennial plants will survive cold temperatures in a certain geographic area. This year, the United States Department of Agriculture released a revised map replacing the 1990 version. The map divides the country into hardiness zones with a 10 degree Fahrenheit difference in average annual minimum temperatures based on data collected in a 30-year period (1976-2005). Zone 1 is the coldest zone (50 below zero to 60 below zero). The zones are further divided into “a and b” with “a” being the colder half of any zone and “b” the warmer.

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The new map is Geographic Information System-based, making it more accurate. It is interactive (by zip code) to improve user experiences, has higher resolution that can show smaller areas of zone delineations. It is designed for web use allowing the user to zoom into a local

area to see the higher temperatures of cities that are heat sinks, the lower temperatures on mountain tops and the buffering effects of large bodies of water on temperature.

In general, the updated map shows us what we have all been experiencing in recent history: warmer low temperatures during winters. Some might argue that this winter, however. A shift of one half of a zone was common for much of the country, including most of Minnesota.

The HZM is based on average minimum temperatures and should only be used as a general guide. For instance, this winter we have had lower temperatures, so pick your plants accordingly. Be cautious in your choices, weighing the risk versus the gain as you trial new perennials. HZM is of no help in predicting plant damage or mortality during acclimation and deacclimation.

Every year, starting in late summer, perennial plants go through a multi-month process called acclimation that prepares them for winter survival. In spring, dormant plants go through a reverse process called deacclimation that restores their ability to actively grow during the growing season. Plants can be winter injured or killed by abnormally low temps during the months of acclimation and deacclimation, too.

HZM provides gardeners with one category of plant performance information: winter survival.

Good plant performance is not just about winter survival. If the new HZM persuades you to plant cultivars and species new to you, remember there are other selection criteria to consider as you match a plant to your planting site: soil texture and moisture, soil pH, light exposure, precipitation and possibly the most important is your own availability to give time and energy to the new planting.

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