Robinia × ambigua Poir. — Garden locust

Introduction

Garden locust is a deciduous woody tree of hybrid origin produced by black locust, Robinia pseudoacacia, and clammy locust, R. viscosa. Its appearance is correspondingly intermediate: plants generally combine the moderate stature, pink flowers, and sticky or viscid young growth associated with clammy locust with stronger stipular spines and some of the vigor of black locust. Plants of the World Online accepts the hybrid and records natural occurrences in western North Carolina, where the native ranges of its parents overlap. (Isely & Peabody 1984; Royal Botanic Gardens, Kew 2026; Trees and Shrubs Online 2026)

The history of the nothotaxon is unusually intertwined with horticulture. The hybrid was known from European cultivation by the early nineteenth century, and cultivated plants helped establish several of its historical names. Natural formation has also been documented within the native range, however, so garden locust should not be understood merely as an artificial cross. The same parental combination can arise independently wherever compatible populations of R. pseudoacacia and R. viscosa meet. (Isely & Peabody 1984; Cierjacks et al. 2013)

The extensive synonymy reflects that horticultural history. Names such as Robinia × dubia, × hybrida, and × bella-rosea have been applied to members of the same hybrid complex. Tree TSAR follows the accepted nothospecific name × ambigua while treating named horticultural selections separately from botanical taxa.

Horticultural and Agricultural Uses

Garden locust has long been valued as an ornamental because it combines the tree form and vigor associated with black locust with the pink flowering inherited from clammy locust. Its flowers and relatively moderate stature have made it especially useful where a pink-flowered locust is desired without the strongly shrubby architecture of some other members of the genus. Trees and Shrubs Online describes the hybrid group as typically showing viscid young shoots and pink flowers, with more developed spines than are usual in R. viscosa. (Trees and Shrubs Online 2026)

Two historically important horticultural selections are ‘Bella-rosea’ and ‘Decaisneana’. The latter has sometimes been placed under R. pseudoacacia, but its pink flowers and slight viscidity have long suggested the influence of R. viscosa. Such cultivars illustrate why botanical hybrid identity and cultivar identity should be kept separate: × ambigua denotes the parental combination, whereas a cultivar represents a particular selected clone within that broader hybrid lineage.

Like its parents, garden locust can sucker and should be given enough space for vegetative spread. Black-locust ancestry also means that old plantings can persist long after deliberate cultivation has ceased. Provenance is therefore useful when interpreting apparently wild occurrences outside the natural contact zone.

Conservation Concerns

Garden locust is not known to require conservation as an independently threatened nothospecies. Its principal conservation value lies in naturally occurring contact zones, especially in western North Carolina, where it records gene flow between R. pseudoacacia and R. viscosa. Because the same hybrid has also been widely cultivated, provenance is essential when distinguishing natural hybridization from horticultural escape.

Additional Information

Plants of the World Online (opens in a new tab): Accepted account for Robinia × ambigua, including parentage, distribution, and synonymy.
Trees and Shrubs Online (opens in a new tab): Horticultural treatment of × ambigua and its principal cultivated forms.
International Plant Names Index (opens in a new tab): Nomenclatural record for × ambigua and associated historical names.

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 × ambigua Poir. Plants of the World Online.

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