Robinia hispida L. — Bristly locust

Introduction

Bristly locust is a suckering deciduous shrub or small tree of the southeastern United States, best known for vivid rose-pink flowers and, in the typical form, long reddish gland-tipped bristles on young stems and inflorescences. The species is centered on the southern Appalachian region, although centuries of cultivation and escape have carried it far beyond its probable native range. Tree TSAR recognizes four varieties within R. hispida: var. hispida, var. fertilis, var. kelseyi, and var. rosea. Dwarf locust, R. nana, is treated separately at species rank. (Isely & Peabody 1984; Weakley & Southeastern Flora Team 2025)

The most important biological fact about R. hispida is that the name-bearing typical variety is not necessarily the most ordinary reproductive condition in the complex. Classical cytological work, later incorporated into the Isely-Peabody revision, showed that typical var. hispida and var. rosea are triploid, whereas var. fertilis and var. kelseyi are diploid. Typical bristly locust is consequently largely sterile and persists mainly by clonal root spread; var. fertilis is striking because it is morphologically close to the type yet sets seed. Tree TSAR retains the infraspecific structure because ploidy and reproductive behavior preserve biological information that broad synonymy would erase. (Whitaker 1934; Isely & Peabody 1984)

The varieties do not all have equal evidentiary security. Var. kelseyi has a long horticultural history but an unresolved natural origin. Isely and Peabody could not establish a convincing wild source, and Wheeler’s genomic study found an unusually strong introgression signal between the single sampled kelseyi accession and black locust. Because that molecular result is based on only one kelseyi individual, Tree TSAR retains the variety provisionally rather than reclassifying it as a nothotaxon. Var. rosea is likewise retained because its triploid biology, morphology, and regional pattern make it more than a simple gradient of hairiness, even though some modern global treatments sink it into broad R. hispida. (Isely & Peabody 1984; Wheeler 2023; Royal Botanic Gardens, Kew 2026)

Horticultural and Agricultural Uses

Bristly locust has been cultivated for centuries for its intensely pink flowers. It spreads readily by suckers when established on its own roots, while horticultural plants have also commonly been grafted onto black-locust rootstocks to create small flowering trees. Grafting can make the plant easier to display as a standard, but shoots from the R. pseudoacacia rootstock can eventually complicate both appearance and identification. The wood is brittle, and exposed specimens can lose branches in storms. (Trees and Shrubs Online 2026)

Propagation and reproductive behavior depend on the taxon being cultivated. The largely sterile triploid forms cannot be treated as ordinary seed-producing shrubs; vegetative propagation is central to their persistence. By contrast, var. fertilis is a seed-producing diploid, and kelseyi is also diploid despite its uncertain origin. This distinction matters for horticultural collections because plants bearing the same broad species label may differ fundamentally in fertility and genetic recombination. (Isely & Peabody 1984)

Conservation Concerns

The broad species is not presently regarded as globally imperiled, but that summary masks substantial variation among its constituent lineages. NatureServe assigns R. hispida a global rank of G4, although the assessment is old and flagged as needing review. Current North Carolina treatment regards the species as secure enough statewide overall while tracking some subordinate varieties as much rarer. Conservation should therefore focus on naturally occurring populations and recognized cytotypes rather than assuming that escaped or cultivated bristly locust represents equivalent genetic diversity. (NatureServe 2026; Vascular Plants of North Carolina 2026)

Clonality makes population counts particularly easy to misread. A visually large patch may represent few genotypes, and a sterile clone can persist for decades without sexual recruitment. Conversely, fertile diploid populations may contain disproportionate evolutionary value because they contribute recombinant seed to a complex otherwise dominated in places by vegetative reproduction. Dense genomic sampling across all four Tree TSAR varieties remains a priority, especially for testing the origin of the triploid lineages and the status of var. kelseyi. (Isely & Peabody 1984; Wheeler 2023)

Additional Information

iNaturalist (opens in a new tab): Observations and photographs of the broad R. hispida concept used by iNaturalist.
Trees and Shrubs Online (opens in a new tab): Horticultural account including historical treatments of the principal varieties.
Plants of the World Online (opens in a new tab): Global name and distribution record; POWO currently includes Tree TSAR varieties and R. nana within broad R. hispida.
Vascular Plants of North Carolina (opens in a new tab): Regional treatment recognizing four R. hispida varieties while separating R. nana.
NatureServe Explorer (opens in a new tab): Global conservation assessment for R. hispida sensu lato.

References and Further Reading

Isely D, Peabody FJ (1984) Robinia (Leguminosae: Papilionoideae). Castanea 49(4): 187-202.

NatureServe (2026) Robinia hispida. NatureServe Explorer. https://explorer.natureserve.org/Taxon/ELEMENT_GLOBAL.2.149692/Robinia_hispida (opens in a new tab)

Royal Botanic Gardens, Kew (2026) Robinia hispida L. Plants of the World Online. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:517689-1 (opens in a new tab)

Trees and Shrubs Online (2026) Robinia hispida. https://www.treesandshrubsonline.org/articles/robinia/robinia-hispida/ (opens in a new tab)

Vascular Plants of North Carolina (2026) Robinia hispida L. North Carolina Biodiversity Project; account based on Weakley’s Flora. https://auth1.dpr.ncparks.gov/flora/species_account.php?id=1512 (opens in a new tab)

Weakley AS, Southeastern Flora Team (2025) Flora of the southeastern United States Web App. Edition of 18 February 2025. University of North Carolina Herbarium, North Carolina Botanical Garden, Chapel Hill. https://fsus.ncbg.unc.edu/ (opens in a new tab)

Wheeler B (2023) Molecular and morphological tests of species delimitation in Robinia of the Southern Appalachians. M.S. thesis, Western Carolina University. https://libres.uncg.edu/ir/wcu/f/Wheeler2023.pdf (opens in a new tab)

Whitaker TW (1934) A karyo-systematic study of Robinia. Journal of the Arnold Arboretum 15: 353-357.