Hi Billjeri, Though I don't have any tree lillies myself I have seen them on garden shows and as far as I am aware they don't need cutting back. They may need a bit of staking but I think normally you just let them die down when flowering is finished. This is certainly the case with the lillies I have in pots which grow to about 5 foot. I let them die right back and then take off the dead "flotsam"...:o)