The automated plant irrigation system only waters the plants if the moisture of the soil is below normal (which means the plants require water!!). The moisture sensor is attached with a panel box (Sensor box) as shown below with an XBee module which transfer data wirelessly, to the main panel box with the relay.
The data transferred from the sensor box (shown above) is the received by another XBee module in the base control panel (as shown below). Arduino in base control panel is programmed to on the relay at a specific time, with the help of real time clock module. So at the specified time, arduino will on/off the relay according to the moisture data received from the sensor unit. This base unit also has NodeMcu module, which is used to send notification to users once the plant is watered.
The notification-sending part is a bit tricky and should be done with much patience :) Notification function uses IFTTT service Webhooks and Notification and the result on smartphone looks like image shown below. For notification function, the user smartphone is required to have the IFTTT mobile application. Other services like SMS in IFTTT doesn't require app installation. And the IFTTT configuration is pretty easy.