Hi Daisy,
to NGA
That depends...
Where is this Iris located?
What kind of Iris is it?
How big is the pot?
Why do you want to keep it in a pot instead of planting it in the ground now?
The biggest thing to remember is that whatever your hardiness zone, your potted plant should be hardy to 2 - 3 zones lower, assuming its in a reasonably sized and insulated pot. That means if you are in zone 8, your iris should be hardy to at least zone 5 or 6.
Daisy
