Hi Vomog, Yes, may be grown in a large pot but what you have is one of the hardiest of the Phormiums and will certainly stand down to about -12 C, infact, you might find it is more hardy planted in the ground, as often even in large tubs, they freeze solid in a bad winter and this wont happen planted in the ground ! Most plants that die in pots, often die from lack of water when they are frozen solid ! If your worried, then dry mulch around the base with straw inside fleece (never plastic) just to protect the basal shoots. The only other thing to note is, that whilst they will thrive in any drained soil, add some grit, just to make sure that the drainage is indeed good, its a fact that the drier you can keep a plant in cold weather, the more cold it will stand !
Lannerman