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The growing season is usually determined by climate and elevation, and in horticulture and agriculture the plant-crop selection. Depending on the location, temperature, daylight hours (photoperiod), and rainfall, may all be critical environmental factors.
In the United States and Canada, the growing season usually means the days between last and first frost, or approximately the last and first occurrence of 0° C / 32° F (freezing) overnight low temperature.
In the northern regions this is roughly May to October, in southern-southwestern-Californian regions it is roughly March to November or longer. Proximity to maritime of extremes can extend the growing season.
In much of Europe, the growing season is defined as the average number of days a year with a 24-hour average temperature of at least 5 °C (6 °C is sometimes used). This is typically from April until October or November, although this varies considerably with latitude and altitude. The growing season is almost year-round in most of Portugal and Galicia, and may be only from June to September in northern Finland and the higher Alps. Proximity to the Gulf Stream and other maritime mediation of extremes can extend the season.
In the United Kingdom, the growing season is defined as starting when the temperature on five consecutive days exceeds 5 °C, and ends after five consecutive days of temperatures below 5 °C. The 1961 to 1990 average season length was 252 days (8.4 months).
In some warm climates, such as the subtropical savanna and Sonoran Deserts or in the drier Mediterranean climates, the growing season is limited by the availability of water, with little growth in the dry season. Unlike in cooler climates where snow or soil freezing is a generally insurmountable obstacle to plant growth, it is often possible to greatly extend the growing season in hot climates by irrigation using water from cooler and/or wetter regions. This can in fact go so far as to allow year-round growth in areas that without irrigation could only support xerophytic plants.
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