Beloved by many gardeners for its heat tolerance and shade-loving nature, but reviled by others for its eagerness to spread with abandon, R. simplex can be divisive.Ruellia is a beautiful addition to a flowering backyard retreat. As a hardy perennial (you only need to plant them once), the Mexican petunia is one of several desert plants that are recommended for warm, dry climates. They are inexpensive, low maintenance, and drought-resistant. Also known as Purple Showers, the Mexican petunia's flower petals are most commonly a vibrant violet color, but they can also be pink or white. Like daylilies, they only bloom once a day, but continuously bloom throughout the summer.