Methods and devices are provided to automatically determine plant water requirements and adjust irrigation in order to make efficient use of water. In one implementation an irrigation control unit comprises at least one input configured to be coupled to and receive signals from a temperature sensor, the signals corresponding to current values of temperature. The unit also includes 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 is configured to determine plant water requirements at least in part using the historical values of the plurality of variables and the current values of the temperature.