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.