The Huorn Leaf is used to create golden Ent Draught, which can be used to hire huorns (by right-clicking saplings with Ent Draught in hand) and summoning them when needed (by drinking the Ent Draught).
For more information on huorns and how to hire them, go to this page.
Some might call it a waste, but this flower can also be used in a regular crafting table or the player's inventory to create yellow dye, alike to other mod flowers.
Black Iris •
Apple Juice •
Berry Juices •