Left alone, Tibouchina tends toward a leggy, open growth habit — long stems reaching for light, flowers clustered mostly at the tips, and a general thinness lower down the plant that only gets worse with age. Regular pruning fixes most of that, and it's one of the lower-effort ways to noticeably improve how the plant looks and flowers.
When to prune
The main pruning window is right after a major flowering flush, before the plant pushes significant new growth. Cutting immediately after bloom gives the plant a full growing season to recover and set up next year's flowers, rather than cutting off wood that was about to bloom. Avoid heavy pruning late in the season in any climate with winter frost risk — fresh growth stimulated by a late cut is far more vulnerable to cold damage than older, hardened wood.
How much to take off
For general shaping, cut back roughly a third of the previous season's growth, cutting just above an outward-facing node to encourage the new shoot to grow away from the center of the plant rather than crossing back through it. For a plant that's gotten seriously leggy and sparse, a harder cut — back by half or more — will set flowering back for a season but usually results in a noticeably denser, better-shaped plant afterward.
Container-grown Tibouchina benefits from slightly more frequent, lighter pruning than one planted in the ground, simply because pot-grown plants have less root volume to support unchecked top growth. A light tip-pruning two or three times through the growing season, on top of the main post-bloom cut, keeps a potted plant compact without requiring one drastic annual cutback.