The growing season is the part of the year during which local weather conditions (i.e. rainfall and temperature) permit normal plant growth. While each plant or crop has a specific growing season that depends on its genetic adaptation, growing seasons can generally be grouped into macro-environmental classes.
Geographic conditions have major impacts on the growing season for any given area. The elevation, or the height above sea level, and temperature of a region are two of the main factors that affect the growing season. Generally speaking, the distance a location is from the equator can be a strong indicator as to what the growing season will look like, however in a high elevation area, regardless of proximity to the equator, a shorter growing season will generally be experienced. Proximity to the ocean also can create less extreme conditions, especially in terms of temperature, which has the potential to extend the growing season further in either direction. In hotter climates, particularly in deserts, despite the geographic barrier of limited water sources, people have been able to extend their growing season in these regions by way of diverting water from other areas and using it in their agriculture. The ability to use these irrigation methods, despite geographic challenges, has made it possible to enjoy almost a year-round growing season.
In the United States and Canada, the growing season usually refers to the time between two dates: the last frost in the spring and the first hard frost in the fall. Specifically, it is defined as the period of time between the average last date at which the overnight low temperature drops below 0 °C (32 °F) in the spring and the average date at which the overnight low first drops down below 0 °C (32 °F) in the fall. These average last and first frost dates have reportedly been occurring earlier and later, respectively, at a steady rate, as observed over the last 30 years. As a result, the overall observed length of the growing season in the United States has increased by about two weeks in the last 30 years.
In the cooler areas of North America, specifically the northern regions of the United States and Canada, the growing season is observed to be between April or May and goes through October. A longer season starts as early as February or March in the west coast and southern regions of the United States and the south coast region of British Columbia, and can continue all the way through November or December. These rough timetables vary significantly for areas that are at higher elevations or close to the ocean.
Because several crops grown in the United States require a long period of growth, growing season extension practices are commonly used as well. These include various row-covering techniques, such as using cold frames and garden fabric over crops. Greenhouses are also a common practice to extend the season, particularly in elevated regions that only enjoy 90-day growing seasons.
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 Spain, and may be only from June to September in northern Finland and the higher Alps. Proximity to the Gulf Stream and other maritime mediations of temperature 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 (8.3 mo).
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. Also in these tropical regions; the growing season can be interrupted by periods of heavy rainfall, called the rainy season. For example, in Colombia, where coffee is grown and can be harvested year-round, they don’t see a rainy season. However, in Indonesia, another large coffee-producing area, they experience this rainy season and the growth of the coffee beans is interrupted.