Ask Mr. Smarty Plants

From a previous Mr. Smarty Plants answer on plants over a septic field:

"So far as we are able to determine, there is no list or database on root lengths of any plants, including natives. The root length of any plant is variable, depending on the plant size, genetics and age as well as environmental conditions. Keep in mind that when your system was designed, it was a well-known fact that something would grow over it-if not planted by you, then weedy volunteers. If those volunteers are not eliminated, some of them are bound to be woody plants, and that's what you're trying to avoid." 

We see no reason why your Ruellia caroliniensis (Carolina wild petunia) would not be fine in the area of the septic field. The plant roots will help to carry moisture up to the surface, and should not interfere with the lines. This plant is a perennial, and annuals are even better because their roots start over every year, but we don't think the Carolina wild petunia would become a nuisance. Follow the plant link above to find out what light and moisture requirements the plant has, bloom time and color, etc. What you do want to avoid is any woody plant, such as trees and shrubs, over or near a septic line because they will definitely cause problems. Another option is native grasses, not necessarily the kind you mow for a lawn, but taller ornamental grasses. Their fibrous roots will help to hold the soil in plant to prevent erosion and should cause no disruption in the septic lines.