Seasonal effects on the cropping-cycle of lettuce in glass-houses during the winter

Current information about the duration of the lettuce cropping-cycles in glasshouses during the winter is presented as linear functions which provide a simple model for crop planning. The data are also plotted against the date in the middle of each cropping-period, in order to show as accurately as possible the effect of growing-conditions on the duration of the cropping-cycle. Plotting the data in this way is based on the concept that the cropping-cycle depends mainly on seasonal radiation, presented by two-sided linear functions with a maximum value around the shortest day (21 December). The amount of radiation appears to be the dominant factor which determines the length of the cropping-cycle.