a combination of two main factors:
Potting mix breaks down, often evidenced
by dead roots, or the plant outgrowing
the container. In the first case, a
larger pot may not be required, simply
replace the growing medium. In the
second case, the plant may need dividing
or may be shifted into a larger pot.
Fresh media should always be used. A
good general rule of thumb is to pot for
the bottom of the plant, the root
system, and not for the top, the foliage.