| Rubiaceae | |
Houstonia nigricans var. angulata
| (Fosberg) W.H. Lewis | Amer. J. Bot. 49: 865 | 1962 |
| Rubiaceae | |
Houstonia nigricans var. floridana
| (Standl.) Terrell | Phytologia 59(1): 79 | 1985 |
| Rubiaceae | |
Houstonia nigricans var. nigricans
| | | |
| Rubiaceae | |
Houstonia nigricans var. papillacea
| (B.L. Turner) Allred | Phytoneuron 2019-59: 2 | 2019 |
| Rubiaceae | |
Houstonia nigricans var. pulvinata
| (Small) Terrell | Phytologia 59(1): 79 | 1985 |