I think you probably could get away with it in a mild zone 7. Personally, I lost interest in this plant as I was sooooo disappointed with its performance as a container grower--it didn't grow (at all) over the summer. What's the secret??

How big was the container? Maybe the plant needs more root room. I am trying P tenax in a 25 gal pot with annuals around it this summer.

I think the big deal with Phormium is drainage - it needs that ever-popular oxymoron, moist, well-drained soil. Standing in wet soil (esp. heavy clay like mine - all the addtives in the world don't make it drain as well as I'd like)will kill it, and its winter hardiness (as is the case with many things) is very dependent on good drainage. To add insult to injury, most cultivars hate hot, humid nights - a problem come July - September here in VA.