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Origin and Habitat: Guanajuato, Hidalgo, Querétaro, San Luis Potosí and possibly Veracruz. Mexico.
Altitude: 3,281-6,070 ft (1,000-1,850 m) above sea level.
Habitat and Ecology: This species is found in xerophyllous scrub on calcareous soils on rocky hills, together with Cephalocereus senilis, Gymnocactus horripilus, Mammillaria schiedeana, Mammillaria wildii, Astrophytum ornatum, Gymnocactus horripilus, Selenicereus spinulosus, Ferocactus glaucescens, Cephalocereus senilis, Stenocereus dumortieri, Fouquieria purpusii, Brahea dulcis, Echeveria coccinea, Tillandsia grandis, Hechtia podantha, Agave stricta and Agave xylonacantha. It forms colonies. In appropriate habitat the species is abundant. Historically the species was overcollected, however today this threat has been reduced.

Description: Mammillaria geminispina is a highly variable densely spined species. Some cluster aggressively and form 30 to 90 cm large clumps that appear white, while other seem to stay more solitary.
Stem: Short cylindrical, light green whit latex, up to 18 cm high and 8 cm in diameter.
Tubercles: Rounded with latex. The axil is filled by white wool and 10 - 20 long white bristles.
Spines: Spines are variable too, some plants have very short while others have long spines.
Central spines: 2 (to 6) pure white or white with dark tip that protrude far out from the rest, straight or slightly curved, 0,5 to 4 cm long (or more).
Radial spines: 16 to 20 chalky white, interlacing up to 7 mm long.
Flowers: Campanulate, pinkish to carmine red, with darker midveins, to 20 mm long and in diameter usually in a ring in the growth of the previous year but sometime more randomly over the body. If grown from seed, M. geminispina can take seven to eight years before flowering.
Fruits: Red
Seeds: Brown.

Notes: Mammillaria elegans A.P. de Candolle 1828. is a 'nomen confusum' also applied to Mammillaria haageana ssp elegans and referes bolth to M. geminispina with latex in the stem (Subgenus Mammillaria section Hydrochylus series supertextae ) and to Mammillaria haageana, without latex in the stem ( Subgenus Mammillaria, section Galactophylous, series Leucocephale)

Cultivation and Propagation: It is a fairly easy plant to grow, don't requires any special treatment, but need as much light as possible without burning the plant to encourage the heaviest spine formation, if kept too dark it may become overly lush and greener and could be prone to rotting due to overwatering.
During the summer it is best to keep the plants outside where the temperature can rise to over 86°F (30°C) with no harm to the plant. Furnish good drainage and use a an open and free draining mineral compost that allows therefore roots to breath. They like only a short winter's rest and should be kept almost completely dry during the winter months, If the soil is allowed to be dry for too long root loss could follow but equally the same result would occur if the plants are both wet and cold. From March onwards the plant will begin to grow and watering should be increased gradually until late May when the plant should be in full growth.
Water regularly during the summer so long as the plant pot is allowed to drain and not sit in a tray of water. During hot weather you may need to water the plants more frequently so long as the plant is actively growing. From late September watering should be reduced to force the plant to go in to a state of semi dormancy, by October you should be back in to the winter watering regime.
Feeding may not be necessary at all if the compost is fresh then, feed in summer only if the plant hasn't been repotted recently. Do not feed the plants from September onwards as this can cause lush growth which can be fatal during the darker cold months. Grown specimens resist to 25°F (-4°C) for a short time, but it is best to keep above 32°F (0°C) to avoid ugly spots on the plant epidermis.
Propagation: Seeds or by stem cuttings from adult plants.

"Mammillaria geminispina" Text available under a CC-BY-SA Creative Commons Attribution License. www.llifle.com 14 Nov. 2005. 07 Mar 2021. </Encyclopedia/CACTI/Family/Cactaceae/14012/Mammillaria_geminispina>

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