LETTUCE (MIXED SALAD LEAVES)

pH: 6.0-7.0
Plant spacing: 18-30 cm (20-25 heads/m3)
Germination time and temperature: 3-7 days; 13-21 °C Growth time: 24-32 days (longer for some varieties) Temperature: 15-22 °C (flowering over 24 °C)
Light exposure: full sun (light shading in warm temperatures) Plant height and width: 20-30 cm; 25-35 cm
Recommended aquaponic method: media bed, NFT and DWC

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Growing lettuce in aquaponic units: Lettuce grows particularly well in aquaponics owing to the optimal nutrient concentrations in the water. Many varieties can be grown in aquaponics, but four main types are included here: crisphead lettuce (iceberg), which has tight head with crispy leaves, ideal for cooler conditions; butterhead lettuce, which show leaves that are loosely piled one on another and have no bitter taste; Romaine lettuce, which has upright and tightly folded leaves that are slow to bolt and are sweet in taste; and loose leaf lettuce, which comes out in a variety of colours and shapes with no head and can be directly sowed on media beds and harvested by picking single leaves without collecting the whole plant. Lettuce is in high demand and has a high value in urban and peri-urban zones, which makes it a very suitable crop for large-scale commercial production.

Growing conditions: Lettuce is a winter crop. For head growth, the night air temperature should be 3-12 °C, with a day temperature of 17-28 °C. The generative growth is affected by photoperiod and temperature extended daylight and warm conditions (> 18 °C) at night cause bolting. Water temperature > 26 °C may also favour bolting and leaf bitterness. The plant has low nutrient demand; however, higher calcium concentrations in water help to prevent tip burn in leaf in summer crops. The ideal pH is 5.8-6.2, but lettuce still grows well with a pH as high as 7, although some iron deficiencies might appear owing to reduced bio-availability of this nutrient above neutrality.
Growing instructions: Seedlings can be transplanted in aquaponic units at three weeks when plants have at least 2-3 true leaves. Supplemental fertilization with phosphorus to the seedlings in the second and third weeks favours root growth and avoids plant stress at transplant. Moreover, plant hardening, through exposing of seedlings to colder temperatures and direct sunlight, for 3-5 days before transplanting results in higher survival rates. When transplanting lettuce in warm weather, place light sun- shade over the plants for 2-3 days to avoid water stress. To achieve crisp, sweet lettuce, grow plants at a fast pace by maintaining high nitrate levels in the unit. When air and water temperatures increase during the season, use bolt-resistant (summer) varieties. If growing in media beds, plant new lettuces where they will be partially shaded by taller nearby plants.
Harvesting: Harvesting can begin as soon as heads or leaves are large enough to eat. If selling to markets, remove the full plants and roots when harvesting as soon as they reach market weight (250-400 g). Cut the roots out and place them in a compost bin. Harvest early in the morning when leaves are crisp and full of moisture and chill quickly.