Origin and Habitat: Mammillaria semperviviSN|20639]]SN|20639]] has a widespread distribution in Central and northern Mexico (Hidalgo, San Luis Potosi, Nuevo Leon, Guanajuato, Queretaro)
Altitude: Grows at altitudes of 1200 to 2250 metres above sea level.
Habitat: This species is rare across its range and occurs in submontane matorral in calcareous soils.

• Mammillaria sempervivi DC.
• Cactus sempervivi (DC.) Kuntze
• Neomammillaria sempervivi (DC.) Britton & Rose

• Mammillaria sempervivi DC.
• Cactus sempervivi (DC.) Kuntze
• Neomammillaria sempervivi (DC.) Britton & Rose
• Mammillaria caput-medusae var. centrispina Salm-Dyck in Labour.
• Mammillaria caput-medusae crassior Salm-Dyck in Labour.
• Mammillaria diacantha Lem.
• Mammillaria sempervivi var. caput-medusae (Otto) Backeb.
• Mammillaria caput-medusae Link & Otto
• Neomammillaria sempervivi var. caput-medusae (Link & Otto) Y.Itô
• Mammillaria sempervivi var. laeteviridis Salm-Dyck
• Mammillaria sempervivi var. tetracantha DC.
• Mammillaria caput-medusae var. tetracantha (DC.) Salm-Dyck
• Mammillaria sempervivi f. tetracantha (DC.) Schelle
• Mammillaria staurotypa Hort.Belg. ex Förster

Description: Solitary at first, or slowly clumping flat with ground.
Stem: Flattened globose to depressed globose and not rising much above ground level, dull grey-green 7.5 cm wide (eventually gets to 10 or 15 cm in diameter)
Tubercles: Pyramidal, firm. The axil is furnished by abundant wooll. Tubercles arrangement 13 - 21
Radial spines: Mostly hidden under white wool, usually on young stems only, 2 - 6, white, bristle-like, to 3 mm long.
Central spines: 2-3, very short, stout, reddish brown to black, becoming grey with age, to 4 mm long (occasionally the lowermost longer).
Roots: Taproots large.
Flowers: Funnelform in a rings on the crown barely rising above the tubercles, whitish to pale yellow with pinkish midstripe, 1-2 cm long and diameter.
Blooming season: Flowering period spring
Fruits: Club-shaped, red, to 10 mm long.
Remarks: The var. caput-medusae and var. tetracantha; often mentioned in literature and sometimes offered in seed lists, should be fully synonymized with the type variety.

Notes: The white wool between the areoles makes this species a pleasure to look at and grow, but the diversity among individual specimens is considerable and the best plants have abundant white axillary wool. Hence, it is worth raising a batch of seedlings and selecting one or two that have the most wool.

Bibliography: Major references and further lectures
1) David Hunt, Nigel Taylor “The New Cactus Lexicon” DH Books, 2006
2) John Pilbeam (1999) “Mammillaria The Cactus File Handbook” Nuffield Press.
3) Nathaniel Lord Britton, Joseph Nelson Rose “Cactaceae: Descriptions and Illustrations of Plants of the Cactus Family” vol. 4 The Carnegie Institution of Washington, Washington 1923
4) Stuart Max Walters “The European Garden Flora: Dicotyledons (Part I)” Cambridge University Press, 1989
5) Edward F. Anderson “The Cactus Family” Timber Press, 2001
6) Guadalupe Martínez, J., Sánchez , E., Bárcenas Luna, R. & Gómez-Hinostrosa, C. 2013. Mammillaria sempervivi. In: IUCN 2013. "IUCN Red List of Threatened Species." Version 2013.2. <www.iucnredlist.org>. Downloaded on 17 January 2014.

Cultivation and Propagation: This is a popular slow growing and easy to cultivate cactus species. Water regularly in summer, but do not overwater (Rot prone) Use pot with good drainage and a very porous potting media, keep dry in winter. Feed with a high potassium fertilizer in summer. It is quite frost resistant if kept dry, hardy as low as -5° C Outside full sun or afternoon shade, inside needs bright light, and some direct sun.
Easily flowered. Some plants will slowly offset , and clumps can be produced in time.
Propagation: Direct sow after last frost.