I grow a lot of Phormiums, and they are very common in my area. However I have no experience with the conditions you reference. Here, we grow them outside. The winter usually doesn't brown the leaves, but in late summer they succumb to heat and dry air and some will brown. Usually the brown is around the outside of the crown, so I just cut those off. Sometimes the tips also brown and I cut those on an angle.

Some people will cut the large ones way back like trimming a shrub. They always recover so you shouldn't worry about trimming, though again, I'm only familar with heathly plants grown in the ground or huge pots.

As with canna_jan, I too only grow outdoors and have no greenhouse experience with Phormium tenax. Is yours the plain green variety P.tenax?

Cheers, Barrie.