Texas is a fairly warm state which lies in USDA hardiness zones 6 to 9. Good flower choices are those which can tolerate the stressful heat of Texas. In most areas of the state, flowers are rotated according to season. The spring-planted annuals like Texas bluebonnets are set out from the end of February to early June; heat-resistant annuals like verbena, portuluca, Mexican sunflower and periwinkle are used in the hot summers; and fall annuals like dianthus and pansies are used in the cooler temperatures of autumn. (Reference 2).
Texas bluebonnet (Lupinus texensis) is not only well suited to the hot and dry climate of Texas, but is also the state flower. The plants are annuals which bloom with spikes of blue flowers from March to April. The flowers are abundant in the rocky sandy soils of hillsides and roadsides and literally blanket the hills, pastures, and prairies of the state in spring. The Texas bluebonnets with their pea like flowers grow optimally in sunny and well drained sites and reach a mature height of 1 to 2 feet. The flowers grow easily from seeds sown during fall and are highly tolerant to drought. Plant seeds ¼ to ½ inch deep in slightly tilled soil and water if there is lack of rain in fall.
Purpletop verbena (Verbena bonariensis) belongs to the verbena plant family and is listed as a of the heat-resistant flower choices for Texas by the Texas Agricultural Extension Service. Verbenas are sprawling plants that bloom with dense clusters of flowers in a variety of colors. They are used frequently for beds, pots and hanging baskets and are hardy plants that are frost tolerant. Purpletop verbena has a straight growth habit and the lavender flowers grow high on wiry stems. The plant is recommended for growing in zones 7 to 11 and reaches a mature height of 3 to 6 feet. Purpletop verbena is ideal companion plants for perennials. Plant in a dry, sunny location. Purpletop verbena is hard to start from seed but easy to root from cuttings.
Mexican sunflowers (Tithonia rotundifolia) are tall summer-blooming annuals with orange-scarlet colored, coarse flowers which attract hummingbirds and butterflies. The plants thrive in dry conditions and intense heat and hence make an ideal summer flower for Texas.They can easily be propagated with seed in zones 5 to 10. Mexican sunflowers reach a mature height of 4 to 7 feet. The flowers are not partial to any particular soil type but grow even better when planted in a moderately fertile soil and full sun. Water during periods of drought and apply liquid fertilizer a couple of times during the growing season. Mexican sunflower is highly resistant to diseases and pests.