What happened is that when you installed the controller you chose the sensor first and the app thought you have the intension to add some standalone sensors (not assigned to any zones in the controller).
To move the stand alone sensors to a zone, first you have to go to Settings->Sensors->Edit(upper right corner) and then delete the sensor. Next you will have to go to the home page. There is a drop down menu and please select your Sprite controller, then go to Add a Product-> Plant sensor and then select the zone and complete the following procedures to add the sensor. You will need to press the button 6 times on the back of the sensor to change its mode to Setup mode (light blinking slowly).