Ask Mr. Smarty Plants

According to this USDA Plant Profile Map, Ruellia drummondiana (Drummond's ruellia) is native to Central Texas, but not to Harris County. Ruellia nudiflora (Violet ruellia), according to this USDA Plant Profile Map, is native to both. They are very similar and it is likely either would grow well in your area. Here are the Growing Conditions from our webpage on Ruellia nudiflora (Violet ruellia):

"Growing Conditions

Water Use: Low , Medium
Light Requirement: Sun , Part Shade , Shade
Soil Moisture: Dry , Moist
Soil Description: Sandy
Conditions Comments: Wild petunia is very easy to grow, spreading vigorously from seed, which in some cases may be problematic. Adds a nice touch to woodland gardens and to walkways. Works well as a groundcover and tolerants mowing. Differs from Drummonds wild petunia because both the flowers and the plant are smaller. Good nectar and larval food source for many butterflies. Cut back after each flush of blossoms to keep compact and blooming all summer."

However, notice these differences in the two sets of Growing Conditions:

Ruellia drummondiana (Drummond's ruellia)

"Growing Conditions

Water Use: Low
Light Requirement: Part Shade , Shade
Soil Moisture: Dry
Soil Description: Rocky soils."

Because the soils and soil moisture in your area are more likely to suit Ruellia nudiflora (Violet ruellia), we suggest you try to stick with that one. Two of our "go-to" sources for seeds, Park Seeds and Native American Seed had no listings for any ruellia at all, so we can see how you could be frustrated. We would like to suggest that if you wait for the first pretty Spring Saturday you make a trip (and go early, so you can get parking) to either a large local commercial nursery or the nursery departments of large home improvement stores and you will find potted, blooming starts of just about whatever you want, including ruellia.

Another possibility is to go to our list of butterfly plants from the National Butterfly Center for South Texas. It does have Ruellia nudiflora (Violet ruellia) on the list but it also has other good native butterfly plants for your area that you might have more luck in finding.