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Firstly, you need to put a two tile tall grass in the tileset you want.
Now, Open up the external editor, or the debug menu (preferably editor though). and Select the option "Edit Terain Tags"
Now find your tiles. The top half should be given terrain tag 10, and the bottom terrain tag 0.
Save and close the editor, and Open up your project.
Open up the Database and go to the tileset that has the tall grass and give the top tile the bush flag and priority 1.
Check out the Example that comes with Essentials, that has tall grass too.