The aims of this study were to automate irrigation in a conventional, bottom-irrigated cabbage seedling bed and to record the irrigation with a volumetric water content sensor on a water-absorbing mat. Two crops were grown in summer, with healthy seedling establishment rates of 91.3% and 88.6%. Growth was normal in the first crop, in which the soil moisture content was always maintained above 20% during germination, but some plants died in the second crop, in which the content dropped to 0% at night owing to excessive drought. Standard practice was to irrigate the beds from sunrise to 10:00 to ~35% water content and then let the soil dry out in the afternoon. Irrigation was also applied in the afternoon on hot days and just before planting. In the second crop, plant death was attributed to insufficient irrigation frequency under increased evaporation due to high temperatures, low humidity, and a high humidity deficit. These results make it clear that simply setting a certain number of irrigations per day is insufficient to account for changes in environment. Instead, it is effective to set a target moisture content for individual time periods and for irrigation to be applied as needed when moisture content is insufficient.