On this page of our game guide to Batman: Arkham Knight you can find a description which explains how to use the Batmobile's sonar to locate Ivy's second plant. From here, you will learn how the sonar can be used in the game.
Drive out of the underground parking lot, activate the Batmobile's Battle Mode and start pressing the Sonar key/button. Each time you use the Sonar look closely at the mini-map, as it will display roots leading to the Ivy's Plant. Follow the roots to the center - marked in the above pictures.
Stop the Batmobile and hold the Sonar button in order to send a special missile under the ground. This will launch another cut-scene.