Elfsong is a plant that is only found in Fangorn forest. It can be used to create yellow Ent Draught, which gives the Regeneration effect, albeit only for one minute. It grows in groups of 3-10 throughout Fangorn.

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A cluster of Elfsong plants.

The plant is renewable as it will occasionally grow when bonemeal is used in Fangorn.

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.

vanilla crafting recipe