BONAP indicates 24 species of Ruellia in the U.S. with 2 species of Ruellia native to Iowa.
Ruellia humilis - Fringeleaf Wild petunia
Ruellia strepens - Smooth petunia
Bonap's maps show more incidents of R. humilis than R. strepens in Iowa. Both populations seem to be entering Iowa from Missouri and from Illinois. Although the plants are variable in their chaeracteristics, Several generalities can be extracted from Steyermark's discriptions of these two plants:
- R. humilis prefers dryer habitats; R. strepens wetter habitats
- R. humilis tends to be hairier (stems hairy on all sides); R. strepens (stems smooth or minutely hairy in 2 bands).
- R. humilis has narrower calyx lobes (less than 2 mm wide) than R. strepens (2-4 mm wide).