And how do I thin it? It's taking over the bed.
Its a phormium. (New Zealand flax)
You can dig it up and split it. It may split your spade first though. They are tough buggers. Thats why the council use them in landscaping a lot. Pretty much vandal proof. Wear gloves while handling the leaves, they are pretty tough and will cut delicate skin.
When is the best time to prune it?
I remove dead leaves in the Spring, as part of the tidy up after winter. You dont prune it as such, if you cut a leaf it will not regrow. If it flowers you can cut the flower stalk down after flowering.