I don’t have much experience with Home-assistant.io (it’s good to know that it’s an open-source home automation platform). If you are interested in starting with some simple control of OpenSprinkler, check the OpenSprinkler API document:
https://opensprinkler.freshdesk.com/solution/folders/5000147084
which describes all supported HTTP GET commands. I assume it’s straightforward to write a script for Home-assistant to open / close a valve using the API.