Why dont you just reduce its size? I didnt think they had roots like trees! I have a phormium that taking off as well! hasnt reached your size yet though.

Cordylines can develop expanding roots like trees, unlike most monocots or true palms, but they are usually not as bad as trees are. I would check the foundation carefully every year, and if there is any sign of cracking or raising, out it comes!

Your Phormium won't reach that size, Maggy7 - it doesn't form a trunk like Cordyline, just spreads sideways through the soil - max height for P. tenax will be about 13 feet at most, and all of it leaf.

Phormium roots don't increase in diameter the way Cordyline roots do, either, so much safer for surrounding pavement and foundations.

Well that is good news lol but I think they must be greedy! as the acer that is in the same raised bed is looking very sorry for itself!!!

Hmm, not sure that's the explanation, Maggy - the conditions Acers like, particularly palmatum varieties (dappled shade, shelter from hot sun and wind) are almost the complete opposite of what Phormiums like (sun, open situation, couldn't care less how windy it is). Chances are your Acer needs to be moved to a more suitable spot.