Abstract: An apparatus for automatically controlling a watering system in response to a comparison of an ambient condition with a time-varying reference level having an adjustable maximum value is disclosed. The invention includes an electrical circuit for monitoring and sensing the magnitude of the ambient condition and for converting the sensed magnitude into a proportional electrical signal. The invention further includes a first timer circuit for generating a triggering signal when the time-varying reference level exceeds the magnitude of the proportional electrical signal. This triggering signal is applied to a second timer circuit which actuates the watering system for a predetermined period of time in response to the triggering signal.