Pot bound Trachycarpus Fortuneii

My six foot high Trachycarpus seems to be thriving, goodness knows how, as it's totally pot bound. There are roots visible everywhere! 

My dilemma is; I really don't want to put it in an even bigger container than it already is in and if I put it in the ground, will it romp away and become a monster? It's about the ideal height now so what I guess I want know is, are Trachys happy in these conditions long term and can I get away with leaving it in situ and ensuring it's well watered with occasional feeding granules scattered around it?

Many thanks ????