Robinia × holdtii Beissn. — Holdt locust

Introduction

Holdt locust is the natural and horticultural hybrid between New Mexico locust, Robinia neomexicana, and black locust, R. pseudoacacia. It is a deciduous woody tree combining the greater stature and vigor of black locust with the pinkish flowers and glandular fruiting structures characteristic of the southwestern parent. Plants of the World Online accepts the nothospecies and records its native hybrid range in Colorado, Utah, and Wyoming. (Isely & Peabody 1984; Royal Botanic Gardens, Kew 2026)

Older horticultural literature often gives the parentage as R. luxurians × R. pseudoacacia. This does not represent a different hybrid concept: Tree TSAR, like modern broad treatments of New Mexico locust, places R. luxurians within R. neomexicana. The historical formula therefore translates directly into the modern parentage R. neomexicana × R. pseudoacacia.

Morphologically, Holdt locust generally resembles a somewhat restrained black locust with looser flower clusters, paler pink or reddish flowers, and glandular pods. The hybrid can be variable because separate crosses between the same parent species need not produce genetically identical offspring. Trees and Shrubs Online traces the name to material introduced from Colorado around 1902, while additional plants with essentially the same parentage had already been raised in European nurseries. (Trees and Shrubs Online 2026)

Robinia × coloradensis Dode is included here as a synonym.

Horticultural and Agricultural Uses

Holdt locust has primarily been cultivated as an ornamental. Its attraction lies in combining a more tree-like habit with pink flowers, creating a visual intermediate between white-flowered black locust and shrubbier western locusts. The cultivar ‘Britzensis’ is among the best-known horticultural representatives of the hybrid complex. It was raised in the Späth nurseries from seed attributed to R. luxurians, the older segregate now included within R. neomexicana. (Trees and Shrubs Online 2026)

The hybrid can attain substantial size, although historical specimens have varied greatly in vigor. As in other Robinia, suckering may occur, and the brittle branch structure characteristic of parts of the genus should be considered when siting mature trees.

Conservation Concerns

Holdt locust is not known to face an independent conservation threat. Naturally occurring plants are nevertheless useful evidence of contact between R. neomexicana and R. pseudoacacia in the interior West. Wild-origin populations should therefore be distinguished from the horticultural material that was distributed widely after the hybrid became known.

Additional Information

Plants of the World Online (opens in a new tab): Accepted account for Robinia × holdtii, including parentage and synonymy.
Trees and Shrubs Online (opens in a new tab): Detailed horticultural history and treatment of ‘Britzensis’.
International Plant Names Index (opens in a new tab): Nomenclatural records for × holdtii and × coloradensis.

References and Further Reading

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

Royal Botanic Gardens, Kew (2026) Robinia × holdtii Beissn. Plants of the World Online.

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