Harlequin glorybower (Clerodendrum trichotomum, Thunb. 1784)

Identity

Harlequin glorybower (Clerodendrum trichotomum Thunb., 1784), also called peanut butter tree or glorytree, is a shrub or small tree in the genus Clerodendrum, family Lamiaceae. POWO, IPNI and GBIF agree on this accepted name, published by Carl Peter Thunberg. ^{1 2 3}

The genus name combines Greek klêros, referring to chance, a lot or the clergy, with déndron, meaning tree. The epithet trichotomum means “three-branched,” referring to the three-forked cymes of the inflorescence. Depending on the interpretation of the Greek root, the name can be read as chance tree or clergy tree. ⁶

Recognition

Young green, four-angled, slightly downy stems develop into a deciduous shrub 2 to 8 m (7 to 26 ft) tall. Opposite, oval, soft leaves commonly measure 10 to 23 cm (4 to 9 in) long. When crushed, they release a recognizable peanut butter odor, a useful identifying feature in the field. ^{4 5}

White, tubular, highly fragrant flowers open in flattened cymes on long stalks in the axils of the upper leaves. Their surrounding green calyx gradually turns red as the fruit ripens. The contrast between white corolla, red calyx and young turquoise drupe is the clearest visual combination for identifying the species. ^{4 12}

Each small turquoise-blue drupe turns dark blue at maturity and remains supported by its persistent red calyx. A plant growing alone flowers normally but generally does not set fruit: the flowers are not self-fertile, and at least two plants are needed to obtain fruit. ^{4 13}

Taxonomy and names

Current classifications in POWO, IPNI and GBIF place Clerodendrum trichotomum in the Lamiaceae, order Lamiales. Some older references, including the University of Copenhagen arboretum account, still use the former family placement, Verbenaceae. ^{1 11}

The World Checklist of Vascular Plants places several historical varieties, including var. fargesii, var. ferrugineum and var. esculentum, in the synonymy of the typical species. Horticultural practice nevertheless continues to treat var. fargesii separately, with its own RHS entries and its cultivar Carnival. ^{1 4}

About 400 species belong to Clerodendrum, most of them tropical. Only two, C. trichotomum and Clerodendrum bungei, were already cultivated as hardy shrubs in pre-Linnaean Europe, one considered lucky and the other unlucky. ¹²

Habitat and range

Central and southern China, Korea, Japan, Taiwan, India and the northern Philippines form its native range. It occupies woodland edges and scrub in humid temperate to subtropical climates. ^{1 11}

European cultivation dates to about 1800. The species has become widespread in British gardens, where two of its forms hold the RHS Award of Garden Merit. ^{11 4}

In the United States, the Russian botanist Karl Maximovich introduced it to Arkansas as early as the 1860s, and it remains widely planted in the Southeast. UF/IFAS nevertheless rates it as a high invasion risk in Florida, with a predictive score of 9 on its scale and a recommendation against planting. Globally, the IUCN assesses it as Least Concern. ^{13 7 8}

Life cycle and flowering

Summer heat brings the first flowers, at a time when few other shrubs still offer color, and flowering continues until the first frosts. Flowers form on the current season’s wood, so spring pruning is compatible with normal flowering in seasonal temperate conditions. ¹²

The drupe’s distinctive blue pigment, trichotomine, was isolated and chemically characterized in 1978. Rare in the plant kingdom, it accounts for the fruit’s unusual turquoise color. ¹⁰

Propagation uses seed or semi-ripe cuttings taken in early summer and rooted under mist. The plant transplants readily and regenerates easily from its roots, which also explains its tendency to send up suckers around the original plant. ^{12 11}

Uses and risks

Fragrance and late flowers are the main reasons for planting this ornamental shrub. It works in informal hedges, as a specimen near a sheltered wall or among other shrubs. Its flowering arrives when relatively few other shrubs are in bloom, a seasonal difference that draws flower-visiting insects. ^{9 11}

No toxic compound is documented for this species in cats, dogs, horses or children, but caution remains appropriate if it is ingested. In parts of East Asia, young leaves are traditionally eaten cooked after boiling to remove their characteristic odor. ¹¹

Its main concern is ecological rather than toxicological. It naturalizes readily and is considered a high invasion risk in Florida by UF/IFAS, while two forms remain RHS award winners in the United Kingdom. Recommendations therefore depend on regional climate and ecological pressure: recognition in British gardens does not override the warning against planting in Florida. ⁷

Varieties, cultivars and look-alikes

Compact growth, bronze young leaves and especially fragrant flowers distinguish variety fargesii (Dode) Rehder from the typical species. It is also reputed to be hardier. Its variegated cultivar Carnival shares its RHS Award of Garden Merit. ⁴

The North American cultivar Stargazer has variegated leaves and yellow-edged flowers, a horticultural label distinct from variety fargesii. ⁶

The closest commonly confused plant is Clerodendrum bungei, the other species of the genus long grown in Europe for its hardiness. Crushed leaves smelling of peanut butter, the persistent red calyx and the turquoise-blue drupe remain the most reliable field marks of C. trichotomum. ^{12 5}

Main sources

• Plants of the World Online / Kew, Clerodendrum trichotomum Thunb.
• International Plant Names Index, Clerodendrum trichotomum Thunb.
• GBIF Backbone Taxonomy, Clerodendrum trichotomum Thunb.
• Royal Horticultural Society, Clerodendrum trichotomum
• NC State Extension Gardener Plant Toolbox, Clerodendrum trichotomum
• Oregon State University Landscape Plants, Clerodendrum trichotomum
• UF/IFAS Assessment of Non-Native Plants in Florida, Clerodendrum trichotomum
• IUCN Red List, Clerodendrum trichotomum assessment (2022)
• Missouri Botanical Garden Plant Finder, Clerodendrum trichotomum
• Iwadare, Shizuri, Yamada & Hirata, 1978, synthesis of trichotomine, Tetrahedron
• University of Copenhagen, Arboretum Hørsholm, Clerodendrum trichotomum plant description
• JC Raulston Arboretum, NC State University, Clerodendrum trichotomum plant profile
• University of Arkansas Division of Agriculture, Plant of the Week, Harlequin Glorybower