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What’s Your Zone?

Plant Hardiness

Q: In the past couple of years, we planted several kinds of evergreen and deciduous trees in our landscape. We carefully checked the hardiness zone maps before planting. We are in zone 5, which is supposed to have extreme minimum winter temperatures of -10 to -20 degrees Fahrenheit.

During the polar vortex at the end of January, our thermometer went to -34 degrees. I realize that the trees may have been harmed, but I am more concerned about why the hardiness zone map was so wrong. We still need to plant more trees. Do I need to look for trees that are hardy to zone 3?

A: I think you have a common misunderstanding about the United States Department of Agriculture Plant Hardiness Zone maps. The maps don’t show the coldest temperature range that your location can get. They show the average coldest temperatures that were achieved during certain years. The current map shows the average coldest temperatures for the years 1976 to 2005, not the coldest temperatures that occurred during those years, or that ever have been in that area, or ever will be in an area.

You would be wise to choose plants that are capable of withstanding the cold of one or more zones farther north than what the map shows. This is more important for trees and large shrubs because they live so long. During their lifetime, they will be exposed to record cold many times.

At the same time, I don’t think you have to worry about your trees too much. Trees growing in each zone have been exposed to the actual extreme temperatures. The only trees that would have a problem would be ones you might have planted that were rated for much warmer zones.

Keep in mind that 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 plant’s survival: heat, street lights versus day length, soil and 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 Salt Lake City, Utah; Oklahoma City, Oklahoma; and Washington, D.C., are in the same cold hardiness zone, they have very different climates when you consider all the other weather factors.

Many gardeners want to grow plants that are not really capable of growing in their landscape for a variety of reasons. For the desired plant, their 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.

Your evergreens that were recently planted and don’t have a very large root system might have needles that turn brown as they dry out in the winter wind. If they green up this spring, they should be fine in other winters when a larger root system can gather more water.

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