Ask Mr. Smarty Plants

Mr. Smarty Plants recently answered a question about the eradication of Ruellia, so I am going to excerpt part of the answer here for you.

The Mexican Petunia (Ruellia brittonia {syn. R. Tweediana}) is a native of Mexico. It can be invasive and is listed as a Category 1 invasive species by the Florida Exotic Pest Plant Council. Its invasiveness is enhanced by its fast growth, prolific production of seeds, and an extensive root system.

Regarding getting it out of your garden, you don't want to spray herbicide in your garden, so one approach is to pull it out, and keep cutting back new sprouts; not necessarily an easy task. You also want to prevent reseeding by removing flowers after they fade so the seed pods won't develop, and also remove new seedlings as they appear. A chemical control method is to use glyphosphate.  Use this very carefully since it can also eliminate all of your other plants. Cut the Petunias down to the ground and apply the glyphosphate to the stumps of the stems. Be sure to follow the directions and warnings on the label.

For more help closer to home, I suggest that you contact the folks at the Tarrant County office of Texas AgriLife Extension.