Robinia viscosa Michx. ex Vent. — Clammy locust

Introduction

Clammy locust is a southern Appalachian shrub or small deciduous tree with pale pink flowers and sticky glandular young shoots, leaf stalks, inflorescences, and fruits. Tree TSAR uses a narrower species concept than Plants of the World Online and Flora of North America by excluding R. hartwigii, which is accepted as a separate southern Appalachian species. Genuine R. viscosa is generally taller and more diffusely viscid, while R. hartwigii is geographically concentrated around high-elevation granitic domes and is characterized by conspicuous stalked glandular trichomes. The distinction is not reducible to a single field character, however, because occasional stalked glands also occur in R. viscosa. Genome-scale analyses provide the stronger evidence that the two represent separate evolutionary lineages. (Wheeler 2023; Weakley & Southeastern Flora Team 2025)

Cultivation complicates the native-range story. Genuine native populations are centered in the southern Appalachians, while planted or escaped clammy locust has been recorded far to the north and outside the region. Plants of the World Online lists an extensive introduced range in eastern North America and Eurasia. Regional botanists likewise caution that roadside and lowland records may represent plantings or escapes. Occurrence maps therefore exaggerate the security of the wild lineage unless provenance is considered. (Royal Botanic Gardens, Kew 2026; Vascular Plants of North Carolina 2026)

Horticultural and Agricultural Uses

Clammy locust has a long ornamental history. Its pink flowers, modest stature, and free flowering made it attractive in European and North American collections, and it has escaped from cultivation widely enough to confuse attempts to reconstruct its original range. The glandular exudate that gives the tree its common name is most obvious on vigorous young growth and inflorescences, although the amount varies among plants and shoots. (Trees and Shrubs Online 2026)

In landscape use, branches can be brittle, suckering can extend a planting beyond its intended footprint, and cultivated material may hybridize with related taxa. Tree TSAR recognizes R. × ambigua as the named R. pseudoacacia × R. viscosa hybrid and R. × longiloba as R. hispida × R. viscosa. Modern southeastern field work has also documented hybrid combinations involving R. hartwigii, reinforcing the reticulate character of the group.

Conservation Concerns

Clammy locust is much less secure in its native habitat than its broad horticultural distribution suggests. NatureServe ranks the species G3, although that global assessment dates from 1999 and is flagged as needing review. North Carolina’s current regional treatment regards genuine native populations as significantly rare, with a state rank around S1-S1S2, and emphasizes uncertainty caused by old plantings and escapes. (NatureServe 2026; Vascular Plants of North Carolina 2026)

Published conservation summaries must also be interpreted taxonomically. Some assessments include Hartwig locust within R. viscosa, whereas Tree TSAR removes it. That means range size, population counts, and threat summaries assembled under a broad clammy-locust concept are not automatically transferable to the narrower Tree TSAR species. Conservation work should voucher populations carefully, document whether occurrences are demonstrably native, and distinguish ordinary clammy locust from R. hartwigii where their ranges approach. (NatureServe 2026; Wheeler 2023)

Open woods, upper slopes, ridges, clearings, and similar disturbance-influenced mountain habitats are important to the species, and succession can reduce suitable open conditions. Protection of wild stands should therefore include both site security and maintenance of the disturbance regime or canopy structure that sustains them. Cultivated abundance elsewhere should not be used as evidence that native southern Appalachian populations require no attention. (NatureServe 2026; Vascular Plants of North Carolina 2026)

Additional Information

iNaturalist (opens in a new tab): Observations and photographs; many records outside the southern Appalachians are cultivated or naturalized.
Trees and Shrubs Online (opens in a new tab): Dendrological and horticultural account; also discusses R. hartwigii under the older varietal treatment.
Plants of the World Online (opens in a new tab): Global taxonomic and distribution record; POWO includes R. hartwigii within R. viscosa.
Vascular Plants of North Carolina (opens in a new tab): Regional account using the narrower species concept adopted by Tree TSAR.
NatureServe Explorer (opens in a new tab): Global conservation assessment; interpret with awareness of its broad taxonomic concept and assessment age.

References and Further Reading

NatureServe (2026) Robinia viscosa. NatureServe Explorer.

Royal Botanic Gardens, Kew (2026) Robinia viscosa Michx. ex Vent. Plants of the World Online.

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

Vascular Plants of North Carolina (2026) Robinia viscosa Michx. ex Vent. 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.