First, are you absolutely certain they are Phormium and not Cordyline or Yucca? There seems to be a great deal of confusion about these plants, so that's why I'm asking. If they're Phormium and still have all the dead, damaged and dying leaves left from the winter, these should be removed at the base.

After all the damage this year, I would also feed them lightly with a low-phosphate food, to help them to recover faster. High-phosphate plant foods--such as Super Bloom, Tomorite, or Miracle Gro for Blooming Plants--will make them very sick.