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Winter hardiness is the ability of a plant to survive the winter conditions in a given location. Cold is an important feature in the winter environment, but several other factors influence hardiness, including soil moisture, relative humidity, and buffeting winds. For example, while a dianthus may tolerate frigid temperature in a garden, it often fails to survive winters where soils stay wet, and although a number of broad-leaf evergreens thrive in cold temperatures, these evergreens may suffer severe desiccation if they are exposed to winter winds.
To assist gardeners with identifying plants that will survive the winter temperature in their gardens, a system of mapping and coding was developed. The USDA Plant Hardiness Zone Map, which was revised and updated in 2001, identifies 15 hardiness zones in the United States according to the average minimum temperatures experienced.
Thousands of plants have been coded to the USDA Plant Hardiness Zone Map according to the low temperatures they will survive. Also considered in the rating is the plant's cold requirement: many plants require a certain amount of cold in order for their buds to break dormancy in the spring. Therefore, the hardiness rating is actually a range from the coldest zone in which the plant will survive to the warmest zone that satisfies its cold requirements.
The influence of cold temperatures on plant survival is more complicated than simply the lowest temperature experienced by the plant. Other factors such as the rate of temperature drop, the duration of the cold, the amount of temperature fluctuation, and the snow or mulch cover on the soil surrounding the plant affect its ability to survive winter conditions.
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