I've used one 5v relais to open and close the valve, a capacitive moisture sensor to measure the watering process in a jar, to handle everything I've used a Wemos. I had to use a 7805 voltage regulator to power everything with a 9V power supply, and I've used a heat sink with the 7805 to reduce the heat.
The code running on the Wemos was written with Arduino IDE.
The App for Android was made with Adobe phonegap, a free tool that lets you make an APK from web files like HTML/CSS/JS.
My home router was used to nat the local ip to a port of the router to call the app from Internet when my smartphone isn't connected to the home wifi.
Here is the schema: