Bioclimatic Modeling of Crisphead Lettuce Phenology As a Tool for Planning Seeding and Harvest

In order to provide a constant supply of crisphead lettuce for the fresh market during the growing season, growers should plan their seeding over several weeks and have to manage their field activities under high climate variability. A bioclimatic model that predicts crop phenological stages in response to temperature and photoperiod would be a useful tool for planning prior to the beginning of the growing season and for monitoring crop development in real time during the growing season. From several experiments in the field, in growth chambers, and in greenhouses, rates of crisphead lettuce seedling emergence, leaf appearance, and head development were established in response to temperature and photoperiod for cultivars 'Ithaca' and 'Estival’. All rates of development have a nonlinear response to temperature. In general, the lower threshold, optimum, and upper threshold temperatures are around 0, 28, and 35°C, respectively, depending on the phenological phase of crisphead lettuce. Short photoperiods slightly reduce the rates of post-emergence phenological phases. These different responses were incorporated into a bioclimatic model that predicts, from sowing to harvest maturity, phenological stages expressed in terms of the modified BBCH scale (Biologische Bundesanstalt, Bundessortenamt und CHemische Industrie) based on weather data measured in the field. For several years and seeding dates per year, the phenological model of crisphead lettuce has generated excellent predictions of different phenological stages observed from sowing to harvest maturity. This tool can be used for helping producers of crisphead lettuce to better manage the impacts of climate variability throughout the whole growing season and to better plan the marketing and distribution of their harvested crops.