Lettuces are seasonal vegetables that originated in India and have been cultivated for more than 2000 years. The world’s largest producers are China and the United States. Their leaves vary in shape and colour depending on the variety. Two of the most popular varieties are cos and iceberg. Cos lettuce is large and upright, while iceberg lettuce is round and smaller.
Lettuce grows well in warm climates, the ideal growing temperature being between 15 and 20ºC. High temperatures can cause early bolting. The plant can withstand moderate frosts of short duration and frequent rainfall causes fungal attack.
Optimum soils for lettuce cultivation are non-saline, well-drained and neutral. It is usually grown in greenhouses using the hydroponic method, achieving high yields per square metre and very clean products. Uniform environmental conditions throughout the production cycle will result in very high quality lettuces.
Planting frames vary according to the lettuce variety. Each variety has different dimensions when ripe. Planting density can vary between 10 and 40 plants per square metre. The planting design should ensure spacing that minimises contact between plants and allows easy access for cultivation. In soil, they are usually planted in a staggered pattern, whereas in hydroponics the planting is adjusted to the design system.
Drip irrigation is the most commonly used system for lettuce irrigation, although there are other systems such as sprinkler and gravity irrigation. This drip irrigation system allows continuous irrigation throughout the crop, maintaining the soil at field capacity throughout the production cycle. The irrigation system should be designed in such a way that all plants on the same ridge receive the same amount of water.
To prevent the collar from rotting, watering should be frequent and in small amounts, making sure that the soil is apparently dry on the surface.