Mammillaria sempervivi a young specimen.

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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.

Scientific Name: Mammillaria sempervivi DC.
First description by De Candolle, Mem. Mus. Hist. Nat. Paris 17: 114 (1828)

Origin:Central and northern Mexico (Hidalgo, San Luis Potosi, Nuevo Leon, Guanajuato, Queretaro)

Habitat: Grows at altitudes of 1200 to 2.250 m.

Conservation status: Listed in CITES appendix 2.

Synonyms:

• Cactus sempervivi (DC.) Kuntze 1891
• Mammillaria bachmannii Boed. ex A.Berger 1929
• Mammillaria caput-medusae Otto ex Pfeiff. 1837
• Mammillaria sempervivi v. caput-medusae (Otto ex Pfeiff.) Backeb. 1961
• Mammillaria tetracentra Otto ex C.F.Först. 1846
• Mammillaria tetracantha

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.
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.

Cultivation: 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.