There are a wide variety of flowering annuals, perennials, vines, shrubs and trees which grow and perform well even when they lack regular watering. Most of these are drought tolerant, which means they can survive without water for extended periods of time.
Salvia (Salvia splendens, S. farinacea) is an annual herb which grows between 10 and 30 inches tall. You'll find Salvia plants that bloom in red, blue or white colors, and they do quite well in full sun to partial shade. Plant them in either spring or summer and they often bloom all season long.
Verbena (Verbena canadensis, V. tensuisecta) is another excellent small flowering plant that does well even without much water. Verbena comes in a wide variety of colors and grows 6 to 12 inches high. It thrives in direct sunlight or partial shade, and will continue blooming from spring or summer throughout the entire season.
Other annuals that do well without much water include Marigolds (Tagetes erecta, T. patula), California Poppy (Escholzia californica) and Sweet Alyssum (Lobularia maritima).
Trumpet Honeysuckle blooms in the spring with yellow, red and orange flowers. It is a fast growing evergreen perennial vine which likes sun to partial shade.
Common Periwinkle (Vinca minor) is a fast growing ground cover which matures to just 5 or 6 inches in height. It has purple flowers in the spring and prefers a bit of shade to grow in.
Other perennials that do well in drought conditions include Purple Coneflower (Echinacea purpurea), Iris (I. kaempferi, I. tectorum, I. danfordae, I. reticulata, I. crestata) and Daylily (Hemerocallis)
Texas Mountain Laurel (Sophora secundiflora) is an evergreen shrub which grows a bit slowly. It is often grown as a shrub of about 5 to 8 feet tall, but can reach heights of 15 to 35 feet in the wild. This shrub has wonderful waxy, leathery leaves and vibrant purple flowers which smell like grape Kool-Aid. The seeds and flowers of this shrub are highly poisonous.
Texas Sage (Leucophyllum frutescens) is a smaller shrub which fully matures to between 5 and 8 feet in height, with a compact 4 to 6 feet of width. This drought tolerant shrub usually produces violet purple flowers but you'll sometimes find them with pink blooms too.
Other low water medium sized shrubs to consider include Wintergreen Barberry (Berberis julianae), Scotch Broom (Cytissus scoparius) and Common Boxwood (Buxus sempervirens).
Crape Myrtle (Lagerstroemia indica) is a fast growing deciduous tree that grows to 20 to 30 feet tall at maturity, and spreads 10 to 15 feet across. This tree grows upgright and prefers full sun.
Many cultivars of the Japanese Flowering Cherry (Prunus serrulata) tolerate low water conditions well too. This medium speed grower can take on oval, weeping and spreading shapes as it matures. Full height measures 20 to 30 feet, and it can spread the equivalent width as well. The Japanese Flowering Cherry is a deciduous tree that likes full sun.
Other trees that have no problem with low water or drought conditions include Bradford Pear (Pyrus calleryana), Saucer Magnolia (Magnolia x soulangiana), American Holly (Ilex opaca) and Flowering Crab (Malus species).