Modular and expandable irrigation controller

A modular irrigation controller comprising:

• a housing:
• a control unit within the housing and comprising a first microcontroller configured to execute stored irrigation programs, wherein the control panel includes an interface connector port that is electrically and communicationally coupled directly to the first microcontroller;
• a backplane within the housing, separate from and coupled to the control unit, the backplane comprising module interface connections configured to receive one or more expansion modules including driver circuitry configured to actuate irrigation valves based on control signals received from the first microcontroller;
• and
• a cartridge removably coupled to the interface connector port, the cartridge comprising a second microcontroller configured to send and receive data communications to and from the first microcontroller, such that the first microcontroller and the second microcontroller function together during use of the modular irrigation controller, wherein the cartridge does not include any driver circuitry configured to actuate any irrigation valves based on any control signals received from the first microcontroller.