I got this relay board [0], then took the power supply from an old sprinkler control system and spliced its leads in. From there, it's dead simple to control using the GPIO python library. I eventually want to monitor things like temperature, soil moisture, and forecast to determine the watering schedule, but right now I just have a cron job that switches the valves on and off a few times a week.

[1] https://www.amazon.com/gp/product/B00KTELP3I