Henequen Leaves in a crafting table create twine. Four pieces of twine can be crafted together to create one string. It is not an efficient process, especially with Flax available.
The seeds can be converted to seed oil, like those of flax. Also in contrast with Flax seeds, Henequen Leaves do not contain aspects, making them useless for Thaumcraft research.
When fully grown, a Henequen plant will change to a darker color. They seem to yield only one leaf at a time.