1. A method for use in irrigation control comprising: receiving, from a user via a user interface of an irrigation control unit and at a time after an initial commercial sale of the irrigation control unit, a region identifier corresponding to a region of the irrigation controller, wherein the irrigation control unit is external to and at a location of an irrigation controller configured to execute an irrigation schedule; identifying, by the irrigation control unit using the region identifier, a set of historical values of one or more environmental variables from a plurality of sets of historical values pre-stored in a memory of the irrigation control unit prior to the initial commercial sale of the irrigation control unit, wherein each of the plurality of sets of historical values corresponds to a different region, wherein the historical values of each of the one or more environmental variables stored in the memory comprise a plurality of historical values corresponding to different time periods; wherein the one or more environmental variables of each of the plurality of sets of historical values are useful in determining plant water requirements, wherein the region identifier corresponds to a region having representative environmental conditions indicated by one of plurality of sets of historical values stored in the memory; receiving, by the irrigation control unit, a current value of at least one other environmental variable useful in determining the plant water requirements from at least one sensor coupled to the irrigation control unit, the at least one other environmental variable being different from the one or more environmental variables and comprising temperature; receiving, by the irrigation control unit, at least one historical value from the identified set of historical values of the one or more environmental variables from the memory; determining, by the irrigation control unit, the plant water requirements using the at least one historical value and the current value of the at least one other environmental variable; outputting, by the irrigation control unit and based on the determined plant water requirements, an adjustment control message to the irrigation controller; and adjusting, by the irrigation controller, execution of the irrigation schedule stored by the irrigation controller in accordance with the adjustment control message, the irrigation schedule defined for a peak irrigation period.