Hardiness of plants describe their ability to survive adverse growing conditions. It is usually limited to discussions of climatic adversity. Thus a plant's ability to tolerate cold, heat, drought, flooding, or wind are typically considered measurements of hardiness. Hardiness of plants is defined by their native extent's geographic location: longitude, latitude and elevation. These attributes are often simplified to a hardiness zone. In temperate latitudes, the term most often describes resistance to cold, or "cold-hardiness," and is generally measured by the lowest temperature a plant can withstand. Hardiness of a plant is usually divided into three categories: tender, half-hardy, and hardy. Tender plants are those killed by freezing temperatures, while hardy plants survive freezing—at least down to certain temperatures, depending on the plant.
Plants vary a lot in their tolerance of growing conditions. The selective breeding of varieties capable of withstanding particular climates forms an important part of agriculture and horticulture. Plants adapt to changes in climate on their own to some extent. Part of the work of nursery growers of plants consists of cold hardening, or hardening off their plants, to prepare them for likely conditions in later life.
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Winter-hardy plants grow during the winter, or at least remain healthy and dormant. Apart from the obvious evergreens, these include many cultivated plants, including some cabbage and broccoli, and all kinds of carrot. Some bulbs – such as tulips – need cold winters in order to bloom, while others – such as freesia – can survive a freezing winter. Many domestic plants are assigned a hardiness zone that specifies the climates in which they can survive. Winter gardens are dependent upon the cultivation of winter-hardy plants.