Nichols and Hanna (1984) report a sporophytic chromosome number of 2n = 72, and a gametophyic number of n = 36 was reported from a plant in Oklahoma (Hardin et al. 1972) and in Queensland, Australia (Sharp 1978, unpubl. data, cited in Symon 1981; P. Sharp 46241 [ADW]).
Solanum dimidiatum is distributed in Texas, Oklahoma, Louisiana, Arkansas, Missouri, and Kansas with a few outlier populations in Illinois, New Mexico, South Carolina, and Mexico. Its native distribution prior to European settlement is not known with certainty because of its weedy, invasive nature and ability to colonize disturbed habitats. It can become a noxious weed locally and has the potential to establish reproducing populations when introduced into suitable habitats. It has been introduced in California and in Australia, but apparently has been successfully eradicated in both areas (Jepson Flora Project 2014; eFloraSA 2014). The California Department of Food and Agriculture rates S. dimidiatum under category 'A' as "a pest of known...environmental detriment" (CDFA 2014). It grows in prairies and oak woodlands as well as disturbed areas such as roadsides, grazed and mowed pastures, ditches, cultivated and urban waste areas, and railroad rights of way in sandy soils or on a variety of other soil types at elevations from 200–600 m.
Solanum dimidiatum is a member of the Carolinense Clade (of subgenus Leptostemonum, Stern et al. 2011), also known as section Lathyrocarpum. Relationships amongst species in the group have been analyzed by Wahlert et al. (2014).
Anderson, H. C. 2002. Calystegine alkaloids in Solanaceous food plants. Copenhagen: Nordic Council.
CDFA. 2014. California Department of Food and Agriculture. http://www.cdfa.ca.gov.
eFloraSA. 2014. Electronic Flora of South Australia. http://www.flora.sa.gov.au.
Hardin, J. W., G. Doerksen, D. Herndon, M. Hobson, and F. Thomas. 1972. Pollination ecology and floral biology of four weedy genera in southern Oklahoma. Southwestern Naturalist 16: 403–412.
Jepson Flora Project. 2014. Baldwin, B. D., D. J. Keil, S. Markos, B. D. Mishler, R. Patterson, T. J. Rosatti, and D. H. Wilken, eds. http://ucjeps.berkeley.edu/IJM.html.
Jordan, J. W. Elisens, and R. Thomas. 2006. Vascular plants utilized by the Plains Apache in southwestern Oklahoma. Publications of the Oklahoma Biological Survey, 2nd Series 7: 24–33.
Menzies, J. S., C. H. Bridges, and E. M. Bailey. 1979. A neurological disease of cattle associated with Solanum dimidiatum. Southwestern Veterinarian 32: 45–49.
Merrill, E. D. 1949. Index rafinesquianus. Jamaica Plains, Massachussetts: The Arnold Arboretum of Harvard University.
Nichols, R. L. and W. W. Hanna. 1984. Irregular meiosis in Solanum dimidiatum. Solanaceae Newsletter 2: 15.
Stern, S., M. F. Agra, and L. Bohs. 2011. Molecular delimitation of clades within New World species of the "spiny solanums" (Solanum subg. Leptostemonum). Taxon 60: 1429–1441.
Symon, D. E. 1981. A revision of the genus Solanum in Australia. Journal of the Adelaide Botanical Garden 4: 1–367.
Wahlert, G. A., F. Chiarini, and L. Bohs. 2014. Phylogeny of the Carolinense clade of Solanum (Solanaceae) inferred from nuclear and plastid DNA sequences. Systematic Botany 39: 1208–1216.
Three common names widely used are Torrey's horsenettle, western horsenettle, and robust horsenettle. Jordan et al. (2006) report that the Plains Apache Native Americans may have utilized the fruits of S. dimidiatum in some aspects of their material culture; however, all parts of the plant are considered poisonous to humans and livestock. In cattle, it is known to cause "crazy cow syndrome," a debilitating disease caused by the calystegine alkaloids present in this species (Menzies et al. 1979; Anderson 2002).
The calculations of extent of occurrence (ca. 760,000 km2) and area of occupancy (1,744 km2) for Solanum dimidiatum were based on its estimated native distribution (i.e., Texas, Oklahoma, Louisiana, Arkansas, Missouri, and Kansas) and excluded the outlying occurrences in Illinois, New Mexico, South Carolina, and Mexico. Given its weedy habit and preference for disturbed habitats, S. dimidiatum is assigned a preliminary conservation status of "least concern" (LC).
In overall habit and floral and fruit morphology, S. dimidiatum is similar to S. perplexum, but it differs by the indumentum on the stems, petioles, leaves, and inflorescence axes [whitish stellate hairs with (4–)6–10 lateral rays, the central ray 1-celled and equal to or shorter than the lateral rays in S. dimidiatum vs. golden stellate hairs with (4–)6–8 lateral rays, the central ray 1–2-celled and longer than the lateral rays in S. perplexum]. It also differs by its smaller prickles on the stems and leaves (up to 6.5 mm vs. up to 15 mm in S. perplexum) and its smaller leaves (up to 16 × 10 cm vs. 22 × 18 cm). Solanum dimidiatum is also somewhat similar to S. carolinense, but can be differentiated by its larger corollas that are up to 4.6 cm in diameter (vs. corollas up to 3 cm in diameter in S. carolinense), its inflorescence that is branched once to several times (vs. unbranched or branched once), and its hard, somewhat dry fruit (vs. a softer, mucilaginous fruit).
Solanum dimidiatum was described by Constantine Samuel Rafinesque, and as is common with Rafinesque names, typification can be problematic because of his characteristically inadequate diagnoses or descriptions, and due to the fact that much of his personal herbarium—including many holotypes—was discarded after his death (Merrill 1949). After a thorough search of herbaria, especially of those with specimens from the Rafinesque herbarium (e.g., P, DWC), we were unable to locate any specimens that would constitute authentic material. However, Rafinesque's description was sufficiently detailed to allow us to confidently match his taxonomic concept to the species commonly identified as S. dimidiatum. We have designated the collection S. Hill 454 from Van Zandt County, Texas as the neotype in order to match the name to a specimen.