Robinia hispida var. fertilis (Ashe) R.T.Clausen — Seedbearing bristly locust

Introduction

Seedbearing bristly locust is a deciduous woody shrub within the unusually complicated Robinia hispida complex. It can be frustratingly difficult to distinguish from typical bristly locust by morphology alone, but its reproductive biology is fundamentally different. Robinia hispida var. fertilis is diploid, with 2n = 20 chromosomes, and regularly produces viable seed. Typical var. hispida, by contrast, is predominantly triploid, with 2n = 30, and fruits poorly or not at all. That contrast between a fertile diploid lineage and a largely sterile triploid lineage was already apparent in Whitaker’s cytological work and became central to Isely and Peabody’s interpretation of the species complex. (Whitaker 1934; Isely & Peabody 1984)

Morphologically, var. fertilis generally falls well within the broader appearance of bristly locust. Its leaflets tend to be relatively narrow, elliptic to oblong-ovate, often acute, and downy beneath, but these characters are less decisive than fertility itself. Trees and Shrubs Online likewise emphasizes seed production as the most important distinction. This biological difference is why Tree TSAR retains var. fertilis even though Plants of the World Online currently includes the name within a broadly circumscribed R. hispida. ITIS and several North American treatments have continued to recognize it at varietal rank.

Important names placed here by Tree TSAR include Robinia fertilis Ashe, R. grandiflora Ashe, R. michauxii Sarg., R. pedunculata Ashe, and R. unakae Ashe. These names reflect the long history of attempts to impose species-level names on localized variants within the bristly-locust complex.

Horticultural and Agricultural Uses

The capacity to reproduce sexually gives seedbearing bristly locust a practical distinction from the predominantly sterile triploid forms of R. hispida. Seed propagation allows genetic recombination rather than merely perpetuating an established clone, although plants can also spread vigorously by root suckers like other members of the genus.

Its rose-pink flowers and shrubby habit give it ornamental value similar to other bristly locusts. Trees and Shrubs Online records the cultivar ‘Monument’ as having been raised from var. fertilis, illustrating its historic use as breeding material as well as a garden plant in its own right.

The combination of suckering growth and seed production can also make the variety useful for rapidly establishing woody cover. Historical northeastern treatments report its use on mine spoil and other disturbed ground, where vegetative spread supplements reproduction from seed. Such behavior also means that plantings should be sited with enough space for colony formation.

Conservation Concerns

The species R. hispida as a whole is not considered globally imperiled, but that broad assessment can obscure the much narrower status of particular cytotypes and varieties. The North Carolina Natural Heritage Program’s 2021 rare-plant list treated var. fertilis as S1 in the state and G4T1Q globally, recording it from acidic cove forests, northern hardwood forests, and high-elevation granitic domes. The “Q” qualifier reflects taxonomic uncertainty, which is appropriate for a lineage distinguished strongly by cytology and fertility but only subtly by morphology. (North Carolina Natural Heritage Program 2021)

Conservation therefore depends on more than simply protecting plants labeled R. hispida. Verified fertile diploid populations represent a different component of the species’ evolutionary diversity from the widespread sterile triploid clones. Provenance documentation, chromosome or reproductive evidence where feasible, and separation of natural populations from old plantings or escapes would all materially improve conservation assessment.

Additional Information

Trees and Shrubs Online (opens in a new tab): Robinia hispida, including var. fertilis.
Plants of the World Online (opens in a new tab): Robinia hispida var. fertilis; treated there as a synonym of R. hispida.
Integrated Taxonomic Information System (opens in a new tab): Robinia hispida var. fertilis.

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. fertilis (Ashe) R.T.Clausen. 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)