Methods and devices are provided to automatically determine plant water requirements and adjust irrigation. In one implementation, a method comprises: receiving, from a user via a user interface of an irrigation control unit and at a time after an initial commercial sale, a region identifier corresponding to a region of the irrigation controller identifying a set of historical values from a plurality of sets of historical values pre-stored in a memory of the irrigation control unit receiving a current value of at least one other environmental variable from at least one sensor receiving at least one historical value from the identified set of historical values from the memory determining the plant water requirements using the at least one historical value and the current value outputting an adjustment control message to the irrigation controller and adjusting execution of the irrigation schedule, the irrigation schedule defined for a peak irrigation period.
