In the United States, Tibouchina is not normally thought of as a houseplant. Indeed, outside of the subtropical zones where it's used as a landscape plant, it's not much thought of at all. I think this is a shame because it's such a beautiful plant with striking purple flowers, silvery and velvety leaves, and an open growth habit.
When it comes to Tibouchina, this is an area where Americans could learn from our overseas friends who do grow Tibouchina.
The plant also goes by the name glory bush or princess flower, perhaps because of its royal purple flowers. In terms of ease, Tibouchina is not terribly difficult, but it is particular. Meet its requirements and you'll have a wonderful, novel houseplant. Annoy it, and you'll have a floor littered with dead leaves and a very unhappy plant.
Light: Bright, unfiltered sunlight. In general, direct summer sunlight is just a bit too strong, but it will not flower correctly without bright light.
Water: Keep it regularly moist in the summer growing season, and reduce water in the winter. However, the plant should never dry completely.
Temperature: Tibouchina flowers in the late spring or mid-summer, when it's warming up or hottest. They do well with mild temperatures, in the mid-70s, and dislike extremes of hot or cold.
Soil: A loose, well-drained potting mix.
Fertilizer: Fertilize during the growing season with liquid fertilizer or controlled-release fertilizer according to label instructions.
Tibouchina can be propagated from semi-woody cuttings, preferably with rooting hormone. The plant is native to Brazil, so cuttings are best held at warm temperatures (around 80˚F) and high humidity. They do best in a propagation house or covered terrarium-type setting. Even then, I still find that Tibouchina cuttings can be difficult to root.
Tibouchina naturally grow to small trees, with a loose, open growth habit ranging up to 20 feet. In the home, the first rule of thumb is to keep Tibouchina closely trimmed by pinching off new growth shoots and gently shaping the plant to contain its sprawling growth. This will likely slow down its growth and reduce the frequency of repotting . When you do repot, go up one pot size and use fresh soil.
Tibouchina belongs to a large genus of plants native to the American tropics, with Brazil having the most. In general, though, only a few species are found in cultivation, and among them, only one is in wide cultivation. This species is T. urvilleana, which goes by several pseudonyms. The plant features four-angled stems, and new growth is covered in orange hairs. The leaves grow to a maximum of about six inches long, with a silver tint. Under ideal conditions, it will bloom all year, but indoors, it's most likely to bloom only in the warmest parts of the year.
I think there are two reasons Tibouchina isn't more widely used. First, it does need some trimming to control its growth habit. Sprawling Tibouchina are leggy and not especially attractive. Second, they seem to have a fairly narrow margin for error: leaf drop and plant decline are unfortunately common, most often because of watering or temperature issues.
In ideal circumstances, Tibouchina thrives in a Mediterranean-like climate, with moderate temperatures and water. If exposed to cold drafts or strong sunlight, expect the plant to start dropping leaves. Tibouchina is not very susceptible to pests.
Even if you have trouble keeping Tibouchina alive over the long haul, they make wonderful display plants for their blooms and will provide a season of color indoors before they decline.