Hi Topakkfn,
Good question,
It looks like the current default landscape material (MI_Creative_Landscape_Apollo_Standard) does not have any way to control the foliage like other landscape materials, and since it is a built-in Fortnite assets there’s no way to edit it to add that layer.
I do want to point out one important thing though - although you’ll see grass poking up from your floor while in-editor, there is a system where in-game the grass is culled.
Are you seeing the grass poking through your floor in-game as well? If yes - are you using a custom/imported asset for your floor?