If it is a woody plant, which it sounds like it may not be, here are some
instructions. I.e., for woody plants.
http://www.treedictionary.com/DICT2003/tree_pruning/

Sincerely,
John A. Keslick, Jr.
Arborist
http://home.ccil.org/~treeman
and www.treedictionary.com
Beware of so-called tree experts who do not understand tree biology.
Storms, fires, floods, earthquakes, and volcanic eruptions keep reminding us
that we are not the boss.

Persephone wrote in message
...

Many of you probably know about, or even have in your garden, this
gorgeous plant that has blue flowers beyond belief.

My two plants are old; have become big & straggly. I've been afraid
to prune, because my gardener told me years ago that if pruned, they
will not regrow.

T or F?

This is what California Gardens site says:

===

With the large (2") passionately purple flowers and felt-like leaves
this Princess Plant is an attention grabber. Tibouchina urvilleana
blooms almost constantly through the spring to frost. Princess Plant
goes winter deciduous for me even in frost free locations. I cut
Tibouchina urvilleana back at during the Winter to pencil sized wood
as most of the smaller branches die anyway. Tibouchina urvilleana is
a coastal gem. It also thrives in the banana belts of the inland
valleys if they are well enough protected. Heat and humidity are both
helpful for Tibouchina urvilleana. The Princess Plant dies back if
temperatures drop into the 20's and way back in the low 20's.
Unfortunately the recovery from the cold is miserably slow. The
biggest Princess Plant that I have seen is 12' tall and wide.
Tibouchina urvilleana prefers loamy well mulched soil.

===

I am in SoCalif Coastal, Zone 24-8

Your input/experience?

Persephone