Acanthocalycium is a small genus of cacti in the family Cactaceae (order Caryophyllales), native to South America — principally Argentina, with species also recorded from Bolivia, Brazil, Paraguay, and Uruguay.
Plants are globose to elongate, typically reaching up to 60 cm in height and 20 cm in diameter. The stems are ribbed — up to 20 ribs per stem — and densely spiny, with areoles bearing numerous spines up to about 8 cm long that are only indistinctly differentiated into marginal and central spines. Plants are usually solitary or branch only sparingly from the base.
The most distinctive feature of the genus is its flowers: funnel-shaped blooms whose outer scales taper into sharp, rigid spines. This spiny floral tube is unique to Acanthocalycium within the cactus family. Flowers range in colour from yellow and orange to white, pink, and magenta, and open during the day. The ovary is protected inside by a ring of hairs.
Acanthocalycium has had a complex taxonomic history: the invalid name Spinicalycium Fric has been synonymised with it, and the genus has periodically been subsumed into the broader genus Echinopsis. Current circumscriptions (including Plants of the World Online) treat it as a distinct genus within Cactaceae.
Etymology
The name Acanthocalycium derives from the Greek akantha ("prickly" or "thorn") and kalyx ("bud" or "calyx"), together referring to the spines that arm the floral tubes — the defining character of the genus.
Distribution
Acanthocalycium is native to southern South America, with its centre of diversity in Argentina. Species are also recorded from Bolivia, Brazil, Paraguay, and Uruguay.
Taxonomy Notes
Acanthocalycium was described with the invalid synonym Spinicalycium Fric, which has since been brought into synonymy. The genus has periodically been included within the broadly circumscribed genus Echinopsis; current treatments by Plants of the World Online recognise it as a distinct genus in Cactaceae.
Cultivation
Acanthocalycium species are cultivated as ornamental cacti. Like most cacti, they require well-draining substrate, full sun to bright light, minimal watering during winter dormancy, and protection from frost. Their showy, colourful flowers make them popular in cactus collections.
