1. An irrigation control unit comprising: at least one input configured to be coupled to and receive signals from a temperature sensor, the signals corresponding to current value of temperature; a memory storing historical values of a plurality of variables; and a processor coupled to the at least one input and the memory, the processor configured to determine plant water requirements at least in part by determining an evapotranspiration (ET) value based at least in part on the historical values of the plurality of variables and the current value of temperature.