An electronic control unit for the control of automatic watering systems having plural lines comprising an electronic circuit with a programmable processing unit for the control of a plurality of different watering lines. A manually operated selector, preferably of the rotary type, is provided, that is settable in a plurality of selectionable positions, to each one of which a specific watering cycle for the lines corresponds. By acting on the selector and on a start button, it is possible to modify the programming of the processing unit in order to associate with each single line a respective watering cycle different from the ones otherwise selectable by means of the selector.