Generally for irrigation system you can use ‘Pump Start Relay’, for example:
These are designed to control high power pumps. OpenSprinkler DC works well with many existing pump start relays such as the above one.
An alternative, low-cost option is solid state relay. Search ‘Solid State Relay’ on Amazon and you can find plenty of choice. Choose one that uses DC input (e.g. 3-32VDC, because you are using OpenSprinkler DC), and controls AC output (e.g. 110VAC should be within the range it controls). Also make sure the solid state relay meets the current rating of your pump. Solid state relays are semiconductor-based so they don’t wear out over time, and they are optically isolated internally.