In reasonable growing conditions in the garden, how large would one of these plants get? Also, would it form a clump that could be divided? (I know they are sterile).

I have one in a pot, but am thinking about planting some in the garden and need to consider spacing. Plus they are really expensive here so I don't want to buy lots and find I could have divided them to make more after a couple of years.

I know your season has passed but it's just starting here (NZ) so I thought I'd ask just in case someone saw my question... :)