[farmergiles]

The more soil they`ve got the better. It`s just a matter of the practicalities- space, money etc. I`m not sure depth is the key so much as volume.

The rule of thumb is that the plants want approx equal room below the soil as they`re taking up above. The roots will tend to go down at first and then spread out sideways when they can go no further down.

When re-potting check if the roots have been growing round and round the base of the pot looking for more room. If they have, tease them out or even cut them otherwise the root growth will be retarded.

You can check the roots by lifting the plant out of the pot carefully (I turn the pot upside down and support the soil with my hand) and visually inspecting them. If the roots look as if they`re cramped up against the sides of the container it`s time to re-pot into a bigger one.