Most Tibouchina in cultivation gets propagated from cuttings, for good reason — it's faster and guarantees the new plant matches the parent exactly. But seed has its place too, particularly if you want a lot of plants cheaply, or you're working with seed collected from a specific plant whose characteristics you're curious about.
Getting seed and starting it
Tibouchina seed capsules form after flowering and should be left on the plant until they dry and begin to split, at which point the tiny seeds inside are ready to collect. They're small enough that handling them individually is impractical — sow them thinly across the surface of a seed-starting mix rather than trying to space them out precisely.
Press them lightly into the surface rather than burying them; Tibouchina seed needs light to germinate well, and covering it with soil tends to reduce germination rates noticeably. Keep the mix consistently moist, provide bright indirect light, and hold the temperature around 21–24°C. Germination is usually visible within two to four weeks, though it can be uneven — don't give up on a tray after the first sprouts appear if others are clearly still coming.
From seedling to garden-ready
Seedlings are slow at first and need to be kept out of direct sun until they've developed several sets of true leaves. Pot up individually once they're large enough to handle without damaging the roots, and expect a considerably longer wait to first flower compared to a rooted cutting — often a full year or more, versus a matter of months for a cutting taken from an already-mature plant. That trade-off, more time for more plants and genuine seed-grown variation, is really the whole case for choosing seed over cuttings in the first place.