Flax is a tile found on the surface. It may also be planted on tilled land and farmed by means of Flax Seeds. Farming Flax is a good way to acquire Fibers, as other sources of fibers aren't renewable.
Flax may be found on the surface, but not in the swamp.
Just like most other farmable plants, Flax may be farmed by placing it's respective seed, Flax Seeds, on tilled land. Over time it will grow and bloom, at which point it may be harvested. When not yet fully grown, a single Flax Seeds is dropped when stepped on by the player.