A shift occurs in leaf crops from field cultivation to cultivation in plastic tunnels and glass greenhouses. As part of the research program ‘Teelt de Grond uit’, funded by Dutch Topsector Horticulture, WUR Greenhouse Horticulture conducted research into the optimization of lettuce cultivation on water in a plastic ‘cabrio’ tunnel greenhouse with LED lighting. High-quality lettuce could be grown, but especially in the case of iceberg lettuce, the growing season was too long in the winter season. The growth of lettuce was stimulated by a higher light intensity of the LEDs, higher room and water temperatures and the addition of a bio-stimulant. A higher plant density gave smaller heads, but more kilos per unit area. In follow-up research in a glass greenhouse, very fast cultivation was realized by applying a lot of LED grow light, high temperatures, humidification and a high CO2 content. The average cultivation time from sowing to harvesting in the dark months of the year was about 40 days. With this cultivation method, 3 times more lettuce heads could be harvested than in a cultivation of heavy lettuce without lighting. A problem was the regular occurrence of tipburn. There were clear differences between lettuce types and cultivars. A very high RH in the night largely prevented the occurrence of tipburn in the final phase of cultivation. The cost price of this cultivation method approaches that of the cultivation of heavy lettuce in the soil.
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