Mammillaria  marcosi is a beautiful plant with white radial spines and dark reddish-brown centrals. It will slowly forms irregular clumps with dense spination. It may grow up to 25 cm in diameter, with up to 30 heads.

Family: Cactaceae (Cactus Family) 

Scientific name:  Mammillaria marcosii W.A.Fitz Maur., B.Fitz Maur. & Glass 1997
Published in:Cactus and Succulent Journal (U.S.).   69(1): 10-14 (1997)

Field collection number: Marcos Sierra 031 (CANTE, holotype; isotypes in MEXU and K).

Origin:  Northeastern Guanajuato (Municipio Atarjea), Mexico. Altitude 1.400m.

Habitat: Grows on volcanic rocks on almost vertical cliffs.

Conservation status: Listed in CITES appendix 2.

Etymology: Named after Marcos Sierra Pichardo, Mexican cactus grower at El Cante, San Miguel de Allende, Guanajuato who discovered this species.

M. marcosi is a beautiful plant with (usually) white radial spines and reddish-brown centrals, but it is also available in a wide range of spine colours. (white, yellow, red and dark-brown)

Description: M. marcosii is a small cactus that readily forms irregular clumps with dense spination.  In habitat the clumps may grow up to 25 cm in diameter, with up to 30 heads. The spines are bicoloured (usually red and white, but they may occur in a wide range of colours)
Stems: Low, round to short cylindrical, up to 5 cm in diameter. Without latex.
Tubercles:  Soft, cylindrical, with round tips. In the axil there is wool and up to 15 bristles, up to 15 mm long.
Radial spines: 16 - 25, radiating, needle-like, 4 - 12 mm long.
Central spines: 7 - 14, mostly needle-like, white,  yellow,  reddish brown or brown, 10 - 20 mm long, one of which is descending and variably hooked, all others straight.
Flower: Campanulated, up to 10 mm in diameter, yellowish or creamy-white, with pink throat and darker pinkish midrib, stigma pale yellow.
Fruit: Red, egg-shaped, 6 - 13 mm long, not arising above the spines.
Seed: Brownish-black.

Cultivation: It is a  slow growing  species of easy culture, recommended for any collection that needs lots of light with ample airflow.  Water regularly in summer, but do not overwater (very wet-sensitively, especially in light of its small root system) Water thoroughly only if soil is dry to the touch, let soil dry in between to prevent root rot, keep dry in winter. Use a very porous mineral-based potting mix, with excellent drainage, as can be achieved by the addition of extra perlite or pumice. Feed with a high potassium fertilizer  in summer. Care must be taken with watering as they tends to become swollen and untidy in growth habit if given too much water and shade.
Sun Exposure: High levels of light are needed to flower and for good spine development.  Can be sunburned if moved from shade/greenhouse into full sun too quickly. During the spring it may be able to take full sun until the heat arrives at the end of spring. In an area that has hot afternoon sun, it may be able to take full morning sun, but requires afternoon shade or afternoon light shade. If grown correctly, it will reward the grower with generous displays of flowers. It is quite frost resistant if kept dry, hardy as low as -5° C (or less) A resting period in winter and strong light are necessary so that it can flower properly.
Clustering in cultivation after several years and easily flowered. For best results, use a shallow pot, and only use the smallest diameter pot that will accommodate the plant. Sensitive to red spider mite. Overhead watering is helpful in controlling mites.

Propagation: Direct sow after last frost (usually) or division, wait until the offsets that appear at the base of old clustered specimens are 1/3 the size of the parent and then detach and plant.