A season is a division of the year marked by changes in weather, ecology, and the amount of daylight. The growing season is that portion of the year in which local conditions (i.e. rainfall, temperature, daylight) 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. Latitude is one of the major factors in the length of the growing season. The further from the equator one goes, the angle of the Sun gets lower in the sky. Consequently, sunlight is less direct and the low angle of the Sun means that soil takes longer to warm during the spring months, so the growing season begins later. The other factor is altitude, with high elevations having cooler temperatures which shortens the growing season compared with a low-lying area of the same latitude.
In agriculture, season extension is anything that allows a crop to be cultivated beyond its normal outdoor growing season. Examples include greenhouses, polytunnels, row cover, and cloches.
The continental United States ranges from 49° north at the US-Canadian border to 25° north at the southern tip of the US-Mexican border. Most populated areas of Canada are below the 55th parallel. North of the 45th parallel, the growing season is generally 4–5 months, beginning in late April or early May and continuing to late September-early October, and is characterized by warm summers and cold winters with heavy snow. South of the 30th parallel, the growing season is year-round in many areas with hot summers and mild winters. Cool season crops such as peas, lettuce, and spinach are planted in fall or late winter, while warm season crops such as beans and corn are planted in late winter to early spring. In the desert Southwest, the growing season effectively runs in winter, from October to April as the summer months are characterized by extreme heat and arid conditions, making it inhospitable for plants not adapted to this environment.
Certain crops such as tomatoes and melons originated in subtropical or tropical regions. Consequently, they require hot weather and a growing season of eight months or more. In colder climate areas where they cannot be directly sowed in the ground, these plants are usually started indoors in a greenhouse and transplanted outside in late spring or early summer.
The Pyrenees and Alps effectively divide Europe into two different regions. The Mediterranean, which is below the 45th parallel, has growing seasons of six months or more and is characterized by hot summers and mild winters. Precipitation mainly falls between October and March, with the summer being dry. In the southern Mediterranean, the growing season is year-round. Mediterranean vegetation is often evergreen due to the mild winters.
Northern Europe ranges from the 45th parallel up past the Arctic Circle. The growing seasons are shorter due to the lower angle of the Sun and generally range from five months to as little as three in Scandinavia and Russia. The Atlantic coast of Europe is moderated considerably by humid ocean air, thus winters are mild and it is rare to see freezing weather or snow. Summers are also mild and as a consequence, many heat-loving plants such as maize will not grow in northern Europe. Further inland, away from the ocean, winters become considerably colder. Despite the short growing season in Scandinavia and Russia, the extreme length of daylight during summer (17 hours or more) allows plants to put on significant growth.
In some warm climates, such as the tropical savanna climates (Aw), the hot semi-arid climates (BSh), the hot desert climates (BWh) or the Mediterranean climates (Cs), 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.
In the 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.