| Author | Nuttall | |
| Distribution | Known only from a very small area in the northeastern Piedmont, essentially over the Durham Diabase Sill in Granville and Durham counties. A now historical report from Wake County.
This is a very widespread species in the Great Plains, from IN and southeastern NE south to LA and TX. However, it is very sporadic in the Eastern states, ranging to southern PA and south to AL. For some odd reason, the BONAP map shows a record for Jackson County, but not for Granville County, which has a handful of long-known specimens.
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| Abundance | Rare in southeastern Granville and the northern half of Durham County, but essentially absent elsewhere. This is a State Threatened species. | |
| Habitat | In NC, this species is restricted to thin, dry, and usually rocky (high pH) soil over diabase rock. It is found in diabase glades and in other clearings, such as powerline clearings, as long as the soil is rather gravelly and high in pH. |
| Phenology | Blooms from May into late summer, and fruits shortly after flowering. | |
| Identification | This is well known to be a highly variable species, with numerous forms, if not necessarily named or accepted varieties or subspecies. And, the form in NC is so different from others that it may represent a good species, or at least an undescribed variety/subspecies. In the state, it is decumbent, branched at ground level, with these stems about 1 foot long at best. The leaves are essentially sessile, but otherwise are like those in R. caroliniensis -- ovate to lanceolate, pubescent, and 2-3 inches long and half as wide. From several axils and branch tips grow a cluster of 4-8 flowers. Each flower has the long floral tube and flared apex, about 1-inch long and slightly less wide at the mouth, normally bright white but a few may be lavender or mixed in color. These flowers average slightly smaller than in other species, but it is the white color that is unique for NC Ruellia. The sprawling, decumbent growth of this species is sufficient in the state for separation from other Ruellia species. Because this is a low-growing species, it can get easily overtopped by many other plants, and thus it does best -- or at least is most easily spotted by an observer -- in very thin soil with little other competition. | |
| Taxonomic Comments | See above. Though all references consider R. humilis as a good species, many do list varieties or subspecies. Weakley (2018) does not.
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| Other Common Name(s) | Low Wild-petunia, Prairie Wild-petunia, Fringeleaf Wild-petunia | |
| State Rank | S1 | |
| Global Rank | G5 | |
| State Status | T | |
| US Status | | |
| USACE-agcp | FACU link |
| USACE-emp | FACU link |
