Robinia hispida var. rosea Pursh — Boynton locust

Introduction

Boynton locust is a southern Appalachian and Piedmont component of the Robinia hispida complex distinguished by a relatively vigorous, often comparatively smooth-stemmed habit. Plants traditionally placed here tend to have longer, more oblong leaflets, more-flowered racemes, and substantially less bristling on the young shoots than typical bristly locust. The entity has repeatedly been elevated to species rank, most notably as Robinia boyntonii, but Tree TSAR follows the more conservative varietal treatment as R. hispida var. rosea. Trees and Shrubs Online likewise retains it provisionally at varietal rank while noting the broader synonymy adopted by POWO.

Its strongest biological distinction is cytological. Like typical var. hispida, var. rosea is triploid, whereas var. fertilis and var. kelseyi are diploid. Triploidy helps explain the poor or irregular production of viable fruit and suggests that Boynton locust cannot be understood simply as a smoother morphological extreme of an otherwise homogeneous species. (Whitaker 1934; Isely & Peabody 1984)

A hybrid origin has occasionally been proposed, particularly involving R. pseudoacacia, but no modern genomic study has tested Boynton locust densely enough to establish such an origin. Tree TSAR therefore treats it as an accepted variety rather than a nothotaxon.

Names assigned here include R. boyntonii Ashe, R. hispida var. boyntonii Ashe, R. albicans Ashe, R. leucantha Rehder, and the historically troublesome macrophylla names. The latter have been applied inconsistently in horticultural literature, making type-based nomenclature more reliable than cultivar labels when interpreting old records.

Horticultural and Agricultural Uses

Boynton locust has obvious ornamental appeal. Its relatively smooth young branches make it easier to handle than the densely bristled typical variety, while the rose-purple flowers are produced in showy clusters during late spring and early summer. Plants corresponding to this phenotype circulated historically under names such as R. boyntonii, R. hispida rosea, and R. macrophylla, contributing to a long history of horticultural nomenclature that does not map perfectly onto modern botanical taxonomy. (Trees and Shrubs Online 2026)

Its triploid condition and limited fruit production make vegetative propagation especially important. Root suckers and root cuttings can preserve established clones, while grafting onto black locust has also been practiced historically for shrubby Robinia. Own-root propagation is preferable where possible because it avoids vigorous R. pseudoacacia suckers emerging below the graft. (Trees and Shrubs Online 2026)

Conservation Concerns

Boynton locust appears to be much more localized than the broad species R. hispida. North Carolina’s 2021 Natural Heritage Program list assigned var. rosea a tentative S2? state rank and G4T3? global rank and associated it principally with open woods in the mountains and Piedmont. The question marks are important: rarity and taxonomic uncertainty are intertwined. (North Carolina Natural Heritage Program 2021)

Its biology also complicates population counting. A large patch can represent vegetative spread from a comparatively small number of genetic individuals, while old ornamental introductions can persist or escape beyond the native distribution. Conservation assessment should therefore distinguish native colonies from cultivated or adventive ones and should avoid equating the number of stems with the number of independent genotypes.

Because both var. rosea and typical var. hispida are triploid, modern genomic comparison of geographically representative colonies would be particularly valuable. Such work could test whether the two forms represent separate clonal lineages, recurrent triploid origins, or parts of a more reticulate evolutionary history.

Additional Information

Trees and Shrubs Online (opens in a new tab): Robinia hispida, including the Boynton-locust treatment.
Plants of the World Online (opens in a new tab): Robinia hispida var. rosea; treated there as a synonym of R. hispida.
Vascular Plants of North Carolina (opens in a new tab): Regional treatment of the R. hispida complex.

References and Further Reading

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

North Carolina Natural Heritage Program (2021) Natural Heritage Program List of Rare Plant Species of North Carolina 2021. North Carolina Department of Natural and Cultural Resources.

Peabody FJ (1984) Revision of the Genus Robinia (Leguminosae: Papilionoideae). PhD dissertation, Iowa State University. https://doi.org/10.31274/rtd-180813-5895 (opens in a new tab)

Royal Botanic Gardens, Kew (2026) Robinia hispida var. rosea Pursh. Plants of the World Online.

Trees and Shrubs Online (2026) Robinia hispida. International Dendrology Society.

Whitaker TW (1934) A Karyo-Systematic Study of Robinia. Journal of the Arnold Arboretum 15(4): 353-357. https://doi.org/10.5962/p.185318 (opens in a new tab)