Robinia × margaretiae Ashe — Rose locust

Introduction

Rose locust is the hybrid between bristly locust, Robinia hispida, and black locust, R. pseudoacacia. It is a deciduous woody tree or large shrub combining the pink to rose-colored flowering of the R. hispida complex with the more vigorous tree habit and other morphological influences of black locust. The hybrid occurs naturally in the southeastern United States and has also arisen repeatedly in cultivation. Plants of the World Online records native occurrences in Georgia, North Carolina, South Carolina, and Tennessee, with introductions farther north. (Isely & Peabody 1984; Royal Botanic Gardens, Kew 2026)

Tree TSAR uses the spelling Robinia × margaretiae rather than × margaretta or × margarettae. Ashe originally used inconsistent forms of the epithet in several genera when honoring Margaret Haywood Henry Wilcox Ashe. Weakley and McCormick’s nomenclatural analysis concluded that these epithets should be corrected and standardized to margaretiae. Tree TSAR follows that interpretation, while retaining the other spellings as search aliases. (Weakley et al. 2020)

Plants of the World Online currently displays the name as R. × margaretta, so readers will encounter a genuine discrepancy between authoritative resources. POWO nevertheless agrees on the biological circumscription and parentage and includes R. × oconeensis and R. × slavinii as synonyms. (Royal Botanic Gardens, Kew 2026)

The inclusion of × slavinii is especially informative. That name was applied to a horticultural cross raised from R. hispida var. kelseyi with R. pseudoacacia as the other parent. Because the evolutionary status of var. kelseyi itself remains unresolved, cultivated material historically called × slavinii may eventually prove genetically more complicated than a simple first-generation cross. At nothospecies level, however, it remains appropriately included within the broader R. hispida × R. pseudoacacia hybrid combination. (Trees and Shrubs Online 2026)

Horticultural and Agricultural Uses

Rose locust is one of the horticulturally important pink-flowered hybrid groups in Robinia. Crossing with black locust can impart greater vigor and a more tree-like habit than is typical of many forms of R. hispida, while the bristly-locust parent contributes rose to lilac-pink flowers. This combination has made hybrid clones attractive as ornamental flowering trees.

Several horticultural selections have historically circulated under the synonym × slavinii. ‘Hillieri’, for example, was described as a small, rounded tree bearing lilac-pink flowers freely even when young. The name × slavinii also remains in horticultural usage despite its inclusion within × margaretiae in current Tree TSAR treatment. (Trees and Shrubs Online 2026)

As elsewhere in the genus, cultivar names should not be confused with botanical nothotaxa. × margaretiae identifies the hispida × pseudoacacia hybrid lineage broadly; selections such as ‘Hillieri’ identify individual horticultural clones derived from within that lineage.

Conservation Concerns

Rose locust is not independently threatened, but natural occurrences help document gene flow between black locust and the biologically complicated R. hispida complex. Provenance is particularly important because horticultural hybrids, including material historically called × slavinii, have been moved widely beyond natural contact zones.

Additional Information

Plants of the World Online (opens in a new tab): Accepted hybrid account under the spelling Robinia × margaretta; lists × oconeensis and × slavinii as synonyms.
Weakley et al. (2020) (opens in a new tab): Nomenclatural treatment supporting the corrected epithet × margaretiae.
Trees and Shrubs Online (opens in a new tab): Horticultural account of × slavinii within the R. hispida treatment.
Royal Horticultural Society (opens in a new tab): Horticultural entry for Robinia × slavinii.

References and Further Reading

Cierjacks A, Kowarik I, Joshi J, Hempel S, Ristow M, von der Lippe M, Weber E (2013) Biological Flora of the British Isles: Robinia pseudoacacia. Journal of Ecology 101: 1623-1640. https://doi.org/10.1111/1365-2745.12162 (opens in a new tab)

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

Royal Botanic Gardens, Kew (2026) Robinia × margaretta Ashe. Plants of the World Online. Tree TSAR treats the epithet as the corrected × margaretiae.

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

Weakley AS, Poindexter DB, Medford HC, Sorrie BA, McCormick CA, Bridges EL, Orzell SL, Bradley KA, Ballard HE Jr, Burwell RN, Lockhart SL, Franck AR (2020) Studies in the vascular flora of the southeastern United States: VI. Journal of the Botanical Research Institute of Texas 14(2): 199-239. https://doi.org/10.17348/jbrit.v14.i2.1004 (opens in a new tab)