Acanthocalycium thionanthum aka Acanthocalycium Thionanthum

Acanthocalycium thionanthum is a small cactus from northwestern Argentina. It belongs to the cactus family, Cactaceae. Sources also name it Echinopsis thionantha and Lobivia thionantha. Its range includes Catamarca, La Rioja, Salta, and Tucumán. LLIFLE also reports a separate site in southern Argentina. It grows on rocky mountain slopes in dry shrubland. Sites lie at 4,921–9,843 feet [1,500–3,000 metres] above sea level. Its habitats include monte shrubland and prepuna.

This succulent subshrub often grows as a single stem. Some plants slowly make new shoots at the base. The round stem takes on a short column shape with age. It reaches about 0.39 feet [0.12 metres] tall, sometimes more. Stem width ranges from 0.20–0.33 feet [0.06–0.10 metres]. The skin ranges from green to deep blue-grey. Older stem sections develop a pale grey, powder-like coat. About nine to fifteen ribs run along each stem.

Raised bumps on the ribs carry the areoles. Young areoles have a coat of yellow-brown felt. Their shape changes as the bumps grow wider. They become oval and lose most of their felt. Each areole has five to ten radial spines. It also has one to four central spines. Spine length, number, and colour vary between wild populations. On one plant, the spines tend to share similar lengths. New spines may look black or light brown. With age, they turn grey or pale yellow-white.

Flowers grow from areoles on the sides of the stem. They have a bell shape and start as hairy buds. Each flower reaches about 0.16 feet [0.05 metres] long or less. Yellow blooms gave the species its name. Yet the species also bears white, orange, or red flowers. The flower tubes carry pale and brown hairs or bristles. Dark scales on the tubes end in spines. Flower colour and spine traits help explain its many named forms. Botanists do not all accept the same divisions within it.

In cultivation, full sun helps keep the stem compact. Sun also supports strong spines and their colour. Too little sun makes stems stretch and stay green. They also fail to form the pale grey coat. People collect this cactus for ornamental use. LLIFLE describes it as abundant at some wild sites. It identifies local collection as a threat to those plants.