Much of the lettuce seed produced in the United States is grown under irrigation in various western states. Few studies have been made to determine the extent to which yields of seed are influenced by soil moisture and other cultural conditions. Crops have been irrigated mostly on the basis of local practices and the general experience of the grower. Recent studies in Utah (Hawthorn 1951) showed rather conclusively that carrot and onion seed crops grown side by side under the same climatic conditions differ widely in their response to soil moisture. High seed yields were obtained from carrots grown under low soil moisture conditions. In contrast high yields of onion seed were obtained only when the soil moisture level was high. However, even with onions, surprisingly satisfactory yields were recorded under low soil moisture conditions. Such findings suggest that desirable and efficient cultural practices for other vegetable seed crops, including lettuce, can only be determined by field experiment.
Hawthorn, Leslie R. and Polland, Leonard H., "Bulletin No. 386 - Production of Lettuce Seed as Affected by Soil Moisture and Fertility" (1956). UAES Bulletins. Paper 345.