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 (100) comprises at least one input (106) adapted to be coupled to and receive signals from a rainfall sensor (110) and a temperature sensor (112), the signals corresponding to current values of an amount of rainfall and temperature. The unit (100) also includes a memory (104) storing historical values of a plurality of variables and a processor (102) coupled to the at least one input (106) and the memory (104). The processor (102) is adapted 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 and the amount of rainfall.
