moss balls

I heard that they grow about 1mm per year
That totally depends on the set-up they are in. Mine are growing WAY faster than that. I think I've had them about 6 months, and my big ones have grown an inch in diameter. The lil' ones have doubled in size, to about an inch.
 
Do NOT bury them. Let them roll, float, play... they are much more interesting this way, and I am fairly certain that any part that is buried will rot.
 
AquariaCentral.com