Cold hardiness determines how well a plant will withstand cold weather. Plants have evolved for different kinds of weather conditions, and have varying degrees of hardiness. Botanists have conducted tests to explore the limits of cold hardiness for a number of species, and this information is readily available from a number of sources, for gardeners who would like to make sure that plants will grow safely in their areas.
There are several different components to cold hardiness. The first thing to understand is that plants are not hardy to the same level year round. Plants which are cold hardy go into a period of dormancy during the winter months which makes it less likely that their tissues will be damaged. This period starts with acclimation, a series of changes which happen in the plant as the days shorten and grow cooler. A cold snap during acclimation can damage or kill a plant because it is not yet ready for cold temperatures.
During midwinter, plants are fully acclimatized and protected. As long as the temperature doesn't drop below the temperature tolerance of the plant, it should survive. Cold hardy plants can live through snow, ice, and other harsh weather conditions. Plants which are less hardy may survive with some damage, or may die off altogether. As the days start to lengthen and warm up, the plant deacclimates and essentially wakes up, producing new growth and getting ready for spring. Again, a cold snap during this period can cause damage to the growing plant.
The cold hardiness of a plant will determine where it can be safely grown. Many governments have published zone maps which divide the nation into various zones based on climate data, with plants being given a zone rating which corresponds to the areas where they can be grown. The USDA zone map is a well-used version, for example, with low numbers in the cold zones to high ones in the hot ones. A plant may be called “hardy to zone four,” or “hardy in zones nine through five,” for example.
Cold hardiness is an important thing to consider when growing plants. People who are gardening in a new area should definitely find out about the temperature ranges in their region and which zone they are in, if a zone map is available. This information can be used at a nursery to select the right plants. Many nurseries specifically stock plants which they know will thrive in their area, but microclimates can be an issue, so gardeners should not assume that a plant in stock at a nursery will necessarily grow in their gardens.