Even though Texas is an expansive state, its southerly location generally ensures long, hot summers. The further east you go, the more ambient humidity accompanies the oppressive heat. Flowering annuals need to be able to handle hot daytime temps as well as sultry overnights--situations that may cause common plants like petunias, impatiens and geraniums to grow too quickly and "fizzle out," especially in July and August.
Butter daisies (Melampodium spp.) may be more commonly called melampodium by nursery workers. These fast-growing annuals grow quickly from seed and form mounds of bright green leaves topped by quarter-sized yellow daisies. After planting butter daisies, expect volunteering, unplanned seedlings to sprout up because the flowers develop many viable seeds.
Moss roses and purslanes (Portulaca spp.) are low, sprawling plants that grow well in sandy, well-draining soils that are moist. Although tolerant of some drought, these plants grow faster and bloom more if soil is moist in the hottest part of summer.
Egyptian Star Cluster
A sure way to attract butterflies to the garden, and hummingbirds if the flowers are more magenta-colored, Egyptian star clusters (Pentas lanceolata) mock Texas summer heat and humidity. Provide lots of sunshine and moist soil for a stupendous crop of flower clusters. Sometimes these annuals are simply called "pentas."
This tropical perennial, sometimes called vinca, is perhaps the best all-around summer annual for Texas' hot summer, especially in the west. Not needing much water, Madagascar periwinkle (Catharanthus roseus) forever blooms in sunny locations up until a frost. It reseeds itself in the landscape, popping up little seedlings this year or next. Its flowers resemble those of impatiens.
While impatiens remain an option for a Texas summer garden, they need tremendous amounts of irrigation to prevent wilting in the long hot days. An alternative for shady garden spaces is the wishbone flower (Torenia fournieri), also called summer pansies or bluewings since they remain low mounding plants and bear trumpet-shaped flowers in tones of blue, purple, violet and pink.