petunia, any plant of the genus Petunia, South American herbs of the family Solanaceae (nightshade family). The common garden petunias, planted also in window boxes, are all considered hybrids of white-flowered and violet-flowered species from Argentina. The plants are of a straggling habit and produce an abundance of large, funnel-shaped, fragrant blossoms. Petunias are classified in the division Magnoliophyta, class Magnoliopsida, order Polemoniales, family Solanaceae.