One of the new features in World of Warcraft Legion is that herbs have ranks now. You can get the new ranks from quest items that you get by picking [Dreamleaf]. Once you finish the quest you get from the herb and the follow up quest, you will get the new rank.
Rank 1 : Gather dreamleaf more effectively.
Rank 2 : You can get [Blood of Sargeras] when gathering dreamleaf.
Rank 3 : Gather even more materials from each dreamleaf plant.

Dreamleaf



Get these three items to speed up farming: [Leystone Hoofplates], [Demonsteel Stirrups], [Stonehide Leather Barding].
You should also enchant your gloves with [Legion Herbalism].
Val'sharah - killing mobsThe best way to farm Dreamleaf is to kill Vilepetal Rooter and gather herbs from their corpse. Defiled Grovewalker also drop Dreamleaf after you use herbalism on their corpse, but the drop rate is a lot lower.

Val'sharah - gathering herbsDreamleaf can only be found in Val'sharah.
Sometimes a Nightmare Creeper will spawn when you pick this herbs, they drop 7-11 Dreamleaf.