Cladrastis shikokiana (Makino) Makino — Yukunoki yellowwood

Introduction

Yukunoki yellowwood (Cladrastis shikokiana) is a Japanese endemic tree of low-elevation forests in the warmer southern half of the country. It reaches about 15 m and bears usually 9–13 leaflets with tapered, often acuminate tips. Drooping flower clusters open in May and June and are particularly characterized by rusty pubescence on their axes. The flattened mature pods lack the broad marginal wings of false-yellowwood (Duley & Vincent 2003; Johnson 2021).

Morphologically, C. shikokiana most closely approaches the Chinese C. wilsonii. The Japanese species generally has somewhat narrower, more lanceolate leaflets and more conspicuously rusty-haired inflorescences. Molecular sampling has been too limited to resolve its exact relationship to the other East Asian yellowwoods with confidence, but the available evidence provides no reason to abandon species rank. Tree TSAR therefore accepts C. shikokiana while retaining its internal placement as a soft dense-sampling question rather than treating the genus itself as unstable (Duan et al. 2019).

The basionym Sophora shikokiana was validly published by Makino in 1900 and transferred to Cladrastis the following year. An earlier 1892 use of the same name lacked the requirements for valid publication and is best treated as historical nomenclatural usage rather than as the basionym date. Current Kew taxonomy accepts the 1900 Sophora name as the homotypic synonym (Royal Botanic Gardens, Kew 2026).

Horticultural and Agricultural Uses

Yukunoki yellowwood is exceptionally uncommon in Western cultivation. The few documented specimens suggest an elegant small to medium tree, but experience is too limited to assign its climatic tolerances with the same confidence available for American yellowwood. Its natural occurrence in low-elevation Japanese forest suggests that provenance will matter greatly when testing plants in colder continental climates (Johnson 2021).

For botanical gardens and specialist collections, wild provenance and accurate vouchers are particularly valuable. A correctly identified accession represents a species that is both poorly represented horticulturally and insufficiently sampled molecularly, giving living collections potential value for future comparative and conservation research.

Conservation Concerns

No global IUCN assessment currently provides an adequate range-wide status for C. shikokiana. Regional Japanese assessments, however, show that the species has undergone serious local contraction. Current compilations list it as Vulnerable in Tokyo and Yamaguchi, Critically Endangered in Fukuoka, Near Threatened in Wakayama, and extinct in Kanagawa; additional regional concern is documented elsewhere in southern Japan (National Red Data Search System 2026).

These regional losses make preservation of known native populations and their forest habitat a higher priority than the absence of a global category might suggest. Geographically documented seed and living collections would also help preserve material for future genomic work capable of resolving population structure and the species’ precise relationship to C. wilsonii.

Additional Information

Plants of the World Online (opens in a new tab): Accepted name, Japanese range, and homotypic synonym Sophora shikokiana.
Trees and Shrubs Online (opens in a new tab): Morphology, habitat, identification, and limited cultivation experience.
National Red Data Search System (opens in a new tab): Compilation of Japanese prefectural conservation assessments for yukunoki.

References and Further Reading

Duan L, Harris AJ, Ye W, Deng S-W, Song Z-Q, Chen H-F, Wen J (2019) Untangling the taxonomy of the Cladrastis clade (Leguminosae: Papilionoideae) by integrating phylogenetics and ecological evidence. Taxon 68(6): 1189–1203. DOI 10.1002/tax.12155 (opens in a new tab).

Duley ML, Vincent MA (2003) A synopsis of the genus Cladrastis (Leguminosae). Rhodora 105: 205–239.

Johnson O (2021) Cladrastis shikokiana. Trees and Shrubs Online.

National Red Data Search System (2026) Cladrastis shikokiana (ユクノキ): Prefectural Red Data designations, Japan.

Royal Botanic Gardens, Kew (2026) Cladrastis shikokiana (Makino) Makino. Plants of the World Online.