Robinia hartwigii Koehne — Hartwig locust

Introduction

Hartwig locust is a localized southern Appalachian shrub or small tree distinguished by pink to rose flowers and conspicuous glandular surfaces. Tree TSAR accepts Robinia hartwigii at species rank, an intentional departure from the current Plants of the World Online and Flora of North America treatment, which place it within R. viscosa. The strongest modern evidence for separation comes from Wheeler’s RAD-seq study: phylogenetic and coalescent species-delimitation analyses supported R. hartwigii and R. viscosa as distinct lineages, while the long-discussed hypothesis that Hartwig locust arose as a hybrid between R. hispida and R. viscosa was not supported. (Wheeler 2023; Royal Botanic Gardens, Kew 2026)

The species is best documented around the Highlands region of southwestern North Carolina, particularly Macon and Jackson counties, where natural populations occur along the margins of exposed granitic domes and in adjacent thin-soiled woods. It is a relatively low woody plant, commonly about 3-4 m tall, and may be difficult to distinguish from clammy locust where only general stature or flower color is considered. The more informative character is its glandular indumentum: Hartwig locust bears prominent multicellular stalked glandular trichomes, whereas the glandular structures of R. viscosa are typically shorter and less distinctly stalked. (Konarska & Łotocka 2020; Weakley & Southeastern Flora Team 2025)

Tree TSAR uses the spelling hartwigii, which is the spelling of Koehne’s original name. The variants hartwegii and hardwegii, and the combination R. viscosa var. hartwigii, remain important search names because they are widespread in floristic and conservation literature. This nomenclatural history matters practically: a conservation record filed under the varietal name may refer to the same biological lineage that Tree TSAR treats as a species. (Royal Botanic Gardens, Kew 2026; Vascular Plants of North Carolina 2026)

Horticultural and Agricultural Uses

Hartwig locust has been cultivated as an ornamental for its compact habit and rose-colored flowers, and cultivated material has also been described as a useful bee plant. Detailed microscopy of plants grown in Poland showed persistent glandular trichomes on shoots, leaves, and inflorescences that produce a complex sticky secretion containing lipids, mucilage, flavonoids, proteins, and alkaloids. Those structures are taxonomically useful as well as visually and tactilely distinctive. (Konarska & Łotocka 2020)

Its native ecology argues for restraint in translating cultivation reports directly into wild-plant management. Genuine native populations occupy exposed granitic settings with thin soils and open to partially open canopies. Plants offered or maintained in cultivation should be documented by provenance where possible, because the horticultural history of Robinia and frequent movement of pink-flowered locusts can obscure the geographic identity of material. Its narrow natural distribution also makes wild collection inappropriate as a source for general horticultural production.

Conservation Concerns

Hartwig locust has the narrowest well-documented range of the six nonhybrid species recognized by Tree TSAR. NatureServe still assesses it under the varietal concept R. viscosa var. hartwigii, with a global rank of G3T2, and reports only a small number of extant occurrences. In North Carolina it is tracked as S1S2 and Significantly Rare. NatureServe identifies forest succession, competition, and land-use disturbance as important pressures, while regional floristic sources emphasize its concentration on high-elevation granitic domes. (NatureServe 2026; Vascular Plants of North Carolina 2026)

The taxonomic mismatch among conservation sources should not be mistaken for biological uncertainty at the same level as it was before genomic sampling. NatureServe’s older hybrid-origin discussion predates Wheeler’s direct test of that hypothesis. Tree TSAR therefore treats habitat protection, population monitoring, and provenance-aware ex situ conservation as measures for an independently evolving species. Maintaining the open or semi-open character of occupied dome-margin habitat is especially relevant where succession is closing the canopy. (NatureServe 2026; Wheeler 2023)

Additional Information

Plants of the World Online (opens in a new tab): Name record; POWO currently treats R. hartwigii as a synonym of R. viscosa.
Vascular Plants of North Carolina (opens in a new tab): Regional species account, distribution, habitat, identification, and conservation status.
NatureServe Explorer (opens in a new tab): Conservation assessment under the name R. viscosa var. hartwigii.
Trees and Shrubs Online (opens in a new tab): Horticultural treatment under R. viscosa var. hartwigii.

References and Further Reading

Konarska A, Łotocka B (2020) Glandular trichomes of Robinia viscosa Vent. var. hartwigii (Koehne) Ashe (Faboideae, Fabaceae) - morphology, histochemistry and ultrastructure. Planta 252: 102. https://doi.org/10.1007/s00425-020-03513-z (opens in a new tab)

NatureServe (2026) Robinia viscosa var. hartwigii. NatureServe Explorer.

Royal Botanic Gardens, Kew (2026) Robinia hartwigii Koehne. Plants of the World Online.

Vascular Plants of North Carolina (2026) Robinia hartwigii Koehne. North Carolina Biodiversity Project.

Weakley AS, Southeastern Flora Team (2025) Flora of the Southeastern United States Web App. University of North Carolina Herbarium, North Carolina Botanical Garden, Chapel Hill.

Wheeler B (2023) Molecular and Morphological Tests of Species Delimitation in Robinia of the Southern Appalachians. MS thesis, Western Carolina University.