The humble petunia is extraordinary for its variety and ease of cultivation.
I'm a fan of the ordinary in the garden, plants that are widely grown because they perform so well. Garden catalogs sing hosannas to obscure species, especially to unheard-of perennials described as having "transparent beauty" or "unusual form and color." In my experience, that often translates as "short-lived," "difficult to grow," and "underwhelming."
Petunias, like marigolds, are among the common flowers that get little respect. Widely adaptable, they are vigorous, self-reliant, and largely pest- and disease-free. They are easy to grow, bloom ceaselessly, and thrive in containers. Add to that their wide availability, their inexpensive price, and their varied forms and colors, and you have the perfect ordinary flower.
While most gardeners treat petunias as annuals, they technically are tender perennials, members of the potato family. Today's hybrids are descendants of two lanky, small-flowered South American species; their seeds were collected nearly 200 years ago and carted to Europe, where German and English breeders crossed them and produced the garden petunia, Petunia x hybrida, with its characteristic five-petal, funnel-shaped flowers.
By 1935 there were double-flowered petunias, thanks primarily to Japanese breeders. Bicolors followed, as did cultivars with larger blooms, a more compact habit, better disease and weather resistance, and new colors, including the first yellow, Summer Sun, released in 1977.
Today there are grandifloras, multifloras, floribundas, millifloras, hedgifloras, supertunias, surfinias, and more. (While technically not petunias, the wildly popular Calibrachoa hybrids, or trailing petunias, look and act like petunias, and are sold under names like Million Bells and Superbells.)