Onion belongs to the Liliaceae family, species Allium cepa L. Onions are cultivated and used around the world. It is most frequently a biennial or a perennial plant, but it is usually treated as an annual and harvested in its first growing season. Modern cultivated types of onion fall into two main groups: the common onion and aggregatum group. Common onions constitute the vast majority of commercial crops and form single bulbs. They include common, dry bulb onions, and salad and pickling onions. Bulb shape, color and leaf characteristics vary markedly within this group. In practice they are categorized according to the day length needed to stimulate bulb development. There are the short ones (12-13 hours) and the intermediate or long (14-16 hours) day length varieties. Types include "Sweet Onions", "Spanish Onions" and "Dutch Onions". Aggregatum onions form clusters of bulbs and include shallots, which have pods similar to garlic and reddish brown skin surrounding a bulb that has subtle flavors of both garlic and onion.
Climate: Onions are widely grown across a range of climates but thrive best when temperatures are cool during early development and then warmer and sunny during maturity. The optimal temperature range for onion cultivation is from 15°C depending on variety.
Soil: Soils need to be well structured, light, deep and rich in organic matter to maximize growth and produce high yields.
pH: normally in the range of 6-6.5
Light: Bulbing onions are day-length sensitive; their bulbs begin growing only after the number of daylight hours has surpassed some minimal quantity. Most traditional European onions are referred to as "long-day" onions, producing bulbs only after 14 hours or more of daylight occurs. Southern European and North African varieties are often known as "intermediate-day" types, requiring only 12–13 hours of daylight to stimulate bulb formation. Finally, "short-day" onions, which have been developed in more recent times, are planted in mild-winter areas in the fall and form bulbs in the early spring, and require only 11–12 hours of daylight to stimulate bulb formation.