Commelinaceae

1. Flowers actinomorphic (radially symmetric), petals blue to purple (rose or white in cultivars). All 6 stamens fertile. Plant perennial. Garden escapes === Tradescantia

Flowers zygomorphic (bilaterally symmetric), all or most petals blue. Stamens: 3 fertile and (2-) 3 sterile or all 6 stamens fertile. Plant annual or perennial. Grain aliens or (more rarely) garden escapes === 2

2. Inflorescence few-flowered, enclosed by folded bract. Stamens: 3 fertile and (2-) 3 sterile. Filaments glabrous === Commelina

Inflorescence with up to 20 flowers in a terminal cyme, well-exerted from the bract. All 6 stamens fertile. Filaments (at least some) densely bearded === Tinantia

Literature:

Faden R.B. (2000) Commelinaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 22. Oxford University Press, New York-Oxford: 170-197.