Mexican Petunia (sometimes called Wild Petunia) is a perennial plant (zones 8 to 10), not at all related to the common annual petunias, known to everyone. Flowers are very showy (especially due to their color) but more so in full sun conditions.
Once established, these plants are draught-resistant but perform much better with regular watering. To maintain their shape, pruning away 1/3 of long stems after every blooming cycle can do wonders. With no pruning, they start looking kind of leggy after a while.
There is one thing about these hardy plants that everyone who wants to grow them in their yard, should be aware of. Wild Petunias can be pretty aggressive. In optimal growing conditions (good soil) they will reseed (this can be prevented by deadheading) and spread by rhizomes. New shoots might suddenly appear several feet away from the mother plant.
On the other hand, Mexican Petunias also make a great indoor container plant if you can spare them a sunny spot. They come in several varieties, with blooms in white, pink or purple-blue.
Wild Petunia 'Katie' dwarf ruellia (or 'Nolan's Dwarf') grows only about 6 inches tall. With its dense growing habit 'Katie' makes a nice ground cover. No deadheading is needed with 'Katie', it blooms all season long and doesn't mind neglect. In good growing conditions it will also reseed and produce true offspring, but it is much less invasive than the rest of its "relatives."
Mexican Petunias are mostly disease and pest-free; only once in a while snails and caterpillars like to munch on them.