Turbinicarpus alonsoi is a small cactus with a single, flattened stem, divided into angled projections known as tubercles. Each tubercle measure up to 1.5 centimetres long and is glaucous (bluish-grey) to grey-green in colour. The areoles of this species bear a reddish-brown ‘wool’, which turns grey with age, and three to five flattened, cardboard-like spines, which measure up to two centimetres long and are irregularly curved inwards. The spines are grey in colour, with darker tips. The flowers of Turbinicarpus alonsoi are borne in the centre of the stem, measure up to three centimetres in diameter, and are rose-magenta in colour (2).
This species somewhat resembles the artichoke cactus, Obregonia denegrii, but the tubercles of the latter are ascending or pressed closely to the stem. It is also similar in appearance to Ariocarpus retusus, which lacks spines, and can be distinguished from the many forms of Turbinicarpus schmiedickeanus by its larger stems and tubercles, and by the magenta flowers (2).
- Diameter: up to 9 cm (2)