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Genus Calliandra in Subfamily Caesalpinioideae

In botanical taxonomy, a genus (plural genera) is a rank used to group closely related species within a family. In the hierarchy, genus sits below family and above species.

Genera are defined by shared morphological, anatomical, and genetic characteristics (for example, features of flowers, fruits, seeds, or leaves) that indicate a close evolutionary relationship among the species they contain.

Each genus can include one or more species. Examples include Rosa (roses) and Solanum (nightshades, including tomato and eggplant).

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Genus Description

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Calliandra (tribe Ingeae, subfamily Caesalpinioideae; Fabaceae) is a medium-sized Neotropical genus with several extra‑American species, estimated at roughly 150 species overall. It occurs from Mexico to northern Argentina, with secondary radiations in Madagascar and the Seychelles, and disjunct taxa on Mascarene islands; it is most diverse in seasonally dry forests and rocky outcrops of the American tropics (Nielsen et al., 2022; LPWG, 2017; POWO, 2024). Calliandra californica is commonly cited as the type species in American treatments (Barneby, 1998).

Morphologically, Calliandra is distinguished by a usually low, often shrubby habit, bipinnate leaves with small glands along the rachis, caducous stipules, and dense axillary fascicles of showy, usually pink to white inflorescences. The flowers are actinomorphic with five small sepals and petals, a conspicuous staminal tube composed of many (typically 20–70) white or pink filaments, and an inferior ovary with axile placentation. The fruit is a dehiscent, flat, elastically coiled legume with compressed seeds. The staminal tube, combined with gland‑bearing leaf rachises and often pale pink filament color, separates Calliandra from close relatives such as Viguieranthus, which bears orange or red filaments and lacks leaf rachis glands (Nielsen et al., 2022).

Centers of diversity lie in the Brazilian Caatinga and adjacent dry forests, the Andes and adjacent Andean foothills, and southern Mexico to Central America; several species are narrow endemics on rocky or limestone substrates. Elevation ranges from near sea level to about 1800 m in montane dry scrub and gallery forests, with pronounced seasonality in rainfall (Nielsen et al., 2022). Pollination is primarily by hummingbirds in American species (Barneby, 1998); dispersal of seeds occurs by explosive dehiscence of the pod. Base chromosome number is well established as x=14 (Iltis, 1953; cited in Barneby, 1998).

Taxonomically, Calliandra is well circumscribed and stable at generic rank, although Malagasy and Seychellois taxa historically placed within it have been transferred to Viguieranthus, a treatment strongly supported by phylogenomic evidence (Nielsen et al., 2022; LPWG, 2017). Within Calliandra, sectional or subgeneric divisions are not widely used, and recent accounts treat the genus as a single lineage within Ingeae (Barneby, 1998). Major shifts in the family include recognition of the “mimosoid clade” within Caesalpinioideae (LPWG, 2017), but these changes do not affect Calliandra’s placement.

Several species (e.g., C. calothyrsus, C. haematocephala, C. tweediei) are widely cultivated as ornamentals and hedgerow plants, valued for showy inflorescences and tolerance of dry soils; C. calothyrsus is a common shade tree in coffee and agroforestry systems (Barneby, 1998). Few taxa are invasive; ecological impacts are localized.

Current threats are unevenly documented, with many narrow endemics lacking assessed statuses, and the genus remains under‑represented in recent conservation assessments (WFO, 2024). Continued targeted surveys and phylogenetic clarification across disjunct populations will be important for conservation and for refining species limits (WFO, 2024).

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