Get into the ‘Zone’ – the USDA Plant Hardiness one

By Dawn Pettinelli - UConn Home & Garden Education Center
Featured Article - posted Tue., Feb. 19, 2013
- Contributed Photo

Will your plants survive the long, cold winter? Whether they do or not depends on their hardiness. Snow, like any other mulch, reduces fluctuations in soil temperature and keeps soil moisture in. So far this year our plants have been exposed to single-digit temperatures in many areas, and have recently experienced the benefit of a thick, snowy blanket.

The term “hardiness” refers to a plant’s ability to survive winter conditions in a given location without special protection. Generally, winter hardiness correlates with the minimum average temperature a plant can withstand. While cold temperatures are very important, other factors also play a role in determining a plant’s hardiness. These include soil conditions, a plant’s vigor going into the winter, the rate of temperature drop, the amount of temperature fluctuations, exposure to sun and drying winds, the length of the winter season, and the presence of snow cover or other mulching material.

If temperatures are unusually cold, or if plants are subjected to other environmental extremes, they may have problems surviving the winter. Often, winter protection in the form of windbreaks, anti-desiccant sprays or mulch can mean the difference between survival and injury or death on harsher sites or of more marginally hardy species.

Different plant parts also exhibit varying degrees of sensitivity to cold. Roots are usually more sensitive to cold temperatures than stems. Flower buds are more tender than leaf buds. This is why some plants that have formed flower buds the previous growing season fail to bloom after a particularly severe winter. The flower buds have been damaged or killed by extreme cold temperatures, yet the leaf buds survive. Often, flower buds that are at the bottom of the stems and were covered by snow come through intact. Plants produce blossoms on the bottom part of the plant that was protected by the snow but not on the exposed upper parts of their stems.

Ever wonder how the stems of trees and shrubs survive winter’s freezing temperatures? Hardy, woody plants have a high proportion of soluble carbohydrates and sugar-like substances in their cells, plus a lower water content than you would find in the cells of tender plants. This cell water solution freezes at a lower temperature than plain water, providing plants with a naturally-occurring “anti-freeze” to help them survive extended periods of freezing weather.

So that you won’t have to guess whether the plants you purchase will be hardy in your garden, the USDA has devised a system for mapping and coding the hardiness of herbaceous and woody species. It is called the Plant-Hardiness Zone Map and was first issued in 1960 and then revised in 1965. It was last updated in 1990 and a new USDA Plant Hardiness Zone Map was released in January of last year. Eleven hardiness zones are identified throughout the United States and Canada, according to minimum winter temperatures experienced. Zones 2 through 10 fall in the continental United States, with Connecticut having areas in zones 4, 5 and 6. The new map is GPS-compatible and have a resolution of 800 square meters, so you can look up the USDA Hardiness Zone of your own backyard!

To check out the USDA Plant Hardiness Zone map, go to: With a little maneuvering, one can find out the plant hardiness zone of any town in Connecticut.

Always check on a plant’s hardiness before adding it to permanent plantings. Selecting plants that are able to withstand our winter conditions is important when choosing perennials or woody ornamental for the landscape. Do keep in mind that other cultural requirements of the plant like exposure, water, soil conditions and the like need also to be met so your plant is in top physical shape to withstand the rigors of winter.

If you have questions about the hardiness zone of a plant you want to purchase or any other gardening query, call the UConn Home & Garden Education Center (toll-free) at 877-486-6271, visit, or get in touch with your local Cooperative Extension Center.

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