University of Vermont Extension 
Department of Plant and Soil Science

Winter News Article 

PLANT HARDINESS: WHAT DOES IT MEAN?

By Dr. Leonard Perry, Extension Professor
University of Vermont

 
Not all plants are hardy in Vermont. Perhaps you learned this the hard way when a favorite shrub or flower didn't make it through the winter.

Hardiness is genetic. That is why some plants are hardier than others, even why some cultivars are hardier than others of the same plant. They may have been bred or selected as being hardier clones.

To further confuse you, plants adapt or change genetically and slowly over time to their climate. That's why a species grown in southern climates may not be as hardy as the same species grown in a northern locale such as Vermont. Keep this in mind when buying plants from mail order companies or nurseries this spring.

Roots, stems, and leaf and flower buds generally are hardy to different temperatures. This is why many perennials die to the ground in winter only to have their roots survive and produce new shoots the following spring.

Forsythias, for example, often have leaves but no flowers. The flower buds, which are less hardy than the leaf buds, are killed by the cold. If flowers appear up to a certain height, but not above this height, this represents the depth of snow cover that protected the flower buds during the killing cold.

So how do you determine plant hardiness?

Start by figuring out your hardiness zone. These are geographic zones shown on maps that share the same range of average annual minimum winter temperatures. A few references also will list hardiness zones for heat--the maximum temperature a plant can endure.

When using a hardiness map, remember that these zones are averages only. In colder years, plants listed as hardy in a particular zone may not survive.

When selecting plants, you need to consider the microclimate--conditions such as soil type, sun and wind exposure, slope, and proximity to buildings. If the soil is heavy, wet, and has low fertility, for instance, it may stress the plant, resulting in winter injury. On the other hand, if the soil is too fertile, the plant may grow late into the season and not harden off properly. Again, the result is winter injury. Adding compost or peat moss will help improve poor soils.

Mulching can help moderate soil temperatures, preventing plant injury. If you select an exposed site where protective snow cover may blow off next winter, remember to mulch your plants late next fall.

Sites exposed to winter winds, especially those from the north or west, can cause desiccation or drying out of evergreens and other ornamentals, resulting in leaf burn. If possible, pick a sheltered site or plan to shield plants with a burlap screen next winter. Tree wrap or guards will help protect trees exposed to early morning winter sun from "frost cracking."

One final thing to consider is that warmer zones can exist within a hardiness zone. Plants near a building may be in a warmer zone due to heat loss from the building or the solar heat absorbed by it. A steep southern slope may be an entire hardiness zone or two warmer than adjacent level areas.


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