An Orion fern tree was a lifeform, a type of tree plant known to be cultivated by the Orion civilization. It was a fern, or resembled one, and was luminous.

A row of luminous Orion fern trees were decorations at a Christmas party at Starfleet Headquarters on Earth in 2271. (TOS - The Lost Years novel: Traitor Winds)

These plants might originate from Rigel II, which has a strong Orion presence and phosphorescent plants, or it might come from the planet Orion.