Plant irrigation system
I've seen so much fake projects from "Viral science". I'm not sure whether he/she simply don't know what he/she does or if he/she simply tries to trap people. This project is a race condition itself ;-).

Besides your battery is empty. I don't think it is a big problem to use a 9V block battery, but Gerben is right, it won't do its job for very long. Have you thought about using a LiPo or LiOn Accu and a Solar loader?

There are worse things I see: 

The sensor is powered by 3.3V but you read the ON/OFF signal with a 5-Volt Pin. That could be a problem, if there are other voltage drops that lead to a signal the PIN can not recognize as HIGH. Check the documentation of the moisture sensor and if it allows 5V use 5V to power it. All sensors I have found on Google have at least a range from 3.3 Volt to 5 Volt. 

Check the current consumption of the relay. It must not draw more current than the Arduino PIN can provide (40mA for the UNO). And, don't use PIN13 to switch the relay. There are so many others and PIN 13 is connected to the onboard LED, which draws 10 - 20 mA on its own.