This feature was added recently, but unfortunately it didn’t make it in time for 4.9, so it’ll be in 4.10. If you’re using source code you could grab the commit here:
https://github.com/EpicGames/UnrealEngine/commit/f9bd6900f4ae11383c8901870c01ebc54eec28d7
It adds a new sub-menu where you can toggle each foliage type independently.