Robinia neomexicana var. rusbyi (Wooton & Standl.) W.C.Martin & C.R.Hutchins ex Peabody — Mogollon locust

Introduction

Mogollon locust is a southwestern geographic variant of New Mexico locust characterized principally by reduced pubescence. It is a rhizomatous woody shrub or small tree with the pink to lavender flowers, pinnate leaves, paired stipular spines, and colony-forming habit typical of Robinia neomexicana. The most useful distinction is on the fruit: plants assigned to var. rusbyi tend toward glabrous or much less hairy pods and generally reduced pubescence elsewhere on the plant. (Isely & Peabody 1984; Felger & Kindscher 2010)

The difference is real but not absolute. New Mexico locust elsewhere can produce fruits that become partly or nearly glabrous, and Felger and Kindscher explicitly described var. neomexicana and var. rusbyi as “weakly differentiated” in the Gila region. That is precisely why Tree TSAR treats Mogollon locust as a variety rather than a separate species.

The variety began nomenclaturally as Robinia rusbyi Wooton & Standl.; R. breviloba Rydb. is also placed here. ITIS continues to recognize var. rusbyi, while Plants of the World Online currently includes it within undivided R. neomexicana.

Despite the Tree TSAR common name, Mogollon locust should not be described as restricted to the Mogollon Rim. The phenotype occurs more broadly in the mountain country of Arizona and New Mexico. “Mogollon locust” is therefore a useful geographic name for the southwestern center of the variety rather than a statement of strict endemism.

Horticultural and Agricultural Uses

In cultivation, Mogollon locust has essentially the same practical attributes as New Mexico locust. It produces showy pink to lavender flowers, tolerates comparatively dry western climates, and spreads from root sprouts to form thickets. In the Gila region, R. neomexicana occurs from pinyon-juniper and oak woodland into mixed-conifer forest and readily colonizes moist, exposed, or disturbed sites.

The reduced pubescence of var. rusbyi is more useful taxonomically than horticulturally. It does not imply a substantially different growth strategy, and plants should generally be sited according to the ecological requirements of New Mexico locust as a whole. Where provenance matters, however, retaining the varietal identity preserves potentially useful information about geographic variation within the species.

Conservation Concerns

There is no strong evidence that Mogollon locust is presently threatened across its range. Robinia neomexicana as a whole is assessed as Least Concern by the IUCN information incorporated into Plants of the World Online, and regional accounts describe New Mexico locust as widespread and common in suitable southwestern mountain habitats.

That broad species-level security should not be interpreted to mean that the variety lacks biological value. The principal conservation significance of var. rusbyi lies in preserving geographic variation within a widespread western species. Because the diagnostic reduction in pubescence grades toward ordinary R. neomexicana, conservation is better served by maintaining multiple native populations across the Arizona-New Mexico landscape than by attempting to protect only a few plants possessing an extreme glabrous phenotype.

Modern population-genomic work could test whether the traditional varietal boundary corresponds to measurable geographic genetic structure. Until such data exist, Tree TSAR’s varietal treatment recognizes a recurring regional phenotype without overstating its evolutionary independence.

Additional Information

Integrated Taxonomic Information System (opens in a new tab): Robinia neomexicana var. rusbyi
Plants of the World Online (opens in a new tab): Robinia neomexicana — includes var. rusbyi within the broader species.
Trees and Shrubs Online (opens in a new tab): Robinia neomexicana
Felger & Kindscher (opens in a new tab): Trees of the Gila Forest Region, New Mexico

References and Further Reading

Felger RS, Kindscher K (2010) Trees of the Gila Forest Region, New Mexico. The New Mexico Botanist, Special Issue 2: 38–66.

Integrated Taxonomic Information System (2026) Robinia neomexicana var. rusbyi (Wooton & Standl.) W.C.Martin & C.R.Hutchins ex Peabody.

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

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

Royal Botanic Gardens, Kew (2026) Robinia neomexicana A.Gray. Plants of the World Online.