Onion is a biennial plant that forms a tunicate bulb. The roots are fasciculate, white and not too deep. The pods that protect the bulb accumulate photo assimilated and thicken, becoming the organ of use of the plant. In the second year the floral thalamus grows and the plant blooms.
In Spain some 23,000 hectares of onion are cultivated, of which almost all of it is cultivated under irrigation. The regions where there is more production are in Levante, Andalusia and Castilla-La Mancha.
It prefers soils of medium or light consistency. It adapts to both moderately alkaline soils and slightly acid soils with a pH of 6 to 6.8. It is a plant that is relatively sensitive to salinity, both in the soil and in the water.
Although onion is a cold resistant plant, its optimum growth temperature is between 12 and 24ºC. At less than 5ºC its growth begins to slow down, and at more than 29ºC it suffers heat damage.
Onions have higher water needs when the bulbs are getting bigger. Once they reach their normal size, the irrigation must be stopped so that the plant dries its air system and the nutrients are concentrated in the bulb.
The different varieties in Spain are grouped depending on the shape and color of the bulbs. There are globose, discoidal, piriform and white, red and even yellow. In Spain There are some popular varieties of Valencian origin, such as Babosa, Medio grano, Liria or Valenciana de Grano. Among others, there is the Blanca Grande de Fuentes, Amarilla de Cabeza Grande Achatada or Roja de Amposta.