'Lucky Bamboo' - what on earth do i do with it?!

What do I do with my 'Lucky Bamboo' (Sorry, but i don't know its proper name!!).

I have 4 stems which are planted up in compost rather than being stood in water, as most people have them. This is because 3 years ago they seemed to be dying off so i asked at a garden centre, who advised me to plant them up. This worked very well as they have thrived ever since.

My problem now is that the plants are getting huge! The stems themselves are no bigger as they had been capped with wax, but the shoots of leaves are so tall that I have had to tie string around them just to prevent the pot toppling over. Can i do anything to reduce the size of the thing as a whole? Otherwise it seems that the shoots will just keep growing and growing. I'm assuming i can prune it somehow but not really sre where to start.

Any help greatly appreciated.

PS, the second pic shows the tips of the new growth, in case that helps?!

 

Your lucky Bamboo is actually not a Bamboo at all. It's latin name is Dracaena sanderiana.