Origin and Habitat: Gymnocalycium nigriareolatumSN|15765]]SN|15765]] occupies a vast area all around the town of Catamarca, known in the Cuesta de los Angeles in the South-West, the Sierra of Graciana in the North, and Cuesta de Portezuelo in the South-east. (typical populations are localized in the zone of the slope of Bethlehem and proximities of the Cuesta de Zapata in Belén)
Habitat and ecology: This species is notably found amongst rocks in clearings and bushes. It is never found growing in places deprived of stones.
Altitude range: 500-1500 metres above sea level.

• Gymnocalycium nigriareolatum Backeb.

• Gymnocalycium hybopleurum (K.Schum.) Backeb. in Backeb. & F.M.Knuth
• Echinocactus multiflorus var. hybopleurus K.Schum.
• Gymnocalycium multiflorum var. hybopleurum (K.Schum.) P.Fourn.
• Gymnocalycium ambatoense Piltz
• Gymnocalycium ambatoense subs. plesnikii Halda & Milt
• Gymnocalycium catamarcense subs. acinacispinum H.Till & W.Till
• Gymnocalycium catamarcense subs. schmidianum H.Till & W.Till
• Gymnocalycium curvispinum var. acuticostatum hort.
• Gymnocalycium nigriareolatum Backeb.
• Gymnocalycium nigriareolatum f. carmineum H.Till
• Gymnocalycium nigriareolatum var. densispinum Backeb. ex H.Till
• Gymnocalycium nigriareolatum var. simoi H.Till

Description: Gymnocalycium nigriareolatumSN|17866]]SN|15765]] has rather strong spines set on darkish areoles, white flowers and bluish fruits. In most of the bibliography it is considered as synonymous of Gymnocalycium hibopleurum, but it has a single bluish-green stem with distinctive black areoles, and dark bluish fruits, unlike of G. hibopleurum, that usually has a clustering habit, clearer areoles and green fruits. Moreover G. nigriareolatum is easily distinguished by the yellow colour of the anthers, which makes it close to Gymnocalycium glaucumSN|15765]]SN|17866]]. The fruit is bluish. The aspect of the plants (length of the spines, shape of the ribs, colour of stems etc..) can vary considerably depending to the local circumstances and in the environment of Cuesta del Portezuelo and Palo Labrado, there are plants intermediate with G. hibopleurum, with green fruits and solitary (not clustering) habit. It could be an intraspecific variation, which seems to be very common in the genus Gymnocalycium. In nature the plants are often attacked by a mould fungus and the areoles develop a characteristic black colouring (hence the species name).
Stem: Solitary rarely with basal shoots ( "var. densispinum"), initially flattened spherical, then sub-spherical, 10-20 cm in diameter, 10-15 cm tall. Glaucous-green to velvety light to dark green reddening in full sun. Apex depressed, woolly, spineless or with young spines.
Root: Napiform, short, strong, often highly branched.
Ribs: 8 to 16(-20), regular, straight, divided by transverse furrows into into angular tubercles up to 3cm wide. The tubercles are rather flat and rounded to distinctly pronounced and sharp-edged, with very prominent conical to angular chins below the areoles.
Areoles: Round to oblong, 7 × 8 mm, at first with white to yellowish brown wool that turns grey or black with time in the wild, whilst new areoles developed in culture remain yellowish-white.
Radial spines: 5-7-(9), radiating, straight to curved toward the body, whitish-grey or pinkish-grey with a brown to black tip, spreading (more comb-shaped in "var. simoi") measuring up to 30 mm long. Young spines black with an orange-brown base.
Central spine: None or one, similar to radials but somewhat longer, with a darker tip and upwards bent when it is present.
Flowers: Near the apex usually white, with a greenish to reddish throat, but sometimes reddish-pink with a darker throat ( "fa. Carmineum"), broadly funnel-shaped, 45-50 mm long, 35-45 mm in diameter Pericarpel short (12 mm), leaf-green, with rounded scales with a white edge and pink point. The scales pass gradually to the external tepals, while elongating and becoming lighter green. External tepals lanceolate, more or less bent towards the outside, white with a median green stripe and a pink top, the median stripe terminates in a brownish spot. Internal tepals are ivory-white, lanceolate. The throat is carmine. Filaments pink. Anthers yellow. Style greenish-white. Stigma yellowish-white, with 10-12 lobes, under the stamens.
Fruits: Light to dark (bluish-) green spherical, vertical, up to 1.5 cm in diameter. The scales have a thin white edge and pink point.
Seeds: Shiny brownish-black, spherical up to 1.1 mm in diameter, the surface covered with minute, wartlike tubercles. Hilum oval, flat, with a spongy edge more or less developed according to the populations.

Bibliography: Major references and further lectures
1) J.G. Lambert “Cacti of Argentina” 2nd edition Revised and supplemented 1997 <https://www.cactuspro.com/biblio_fichiers/pdf/Lambert/LambertEN.pdf>
2) Gymnocalycium nigriareolatum Backeb. <http://www.atlas.catamarca.gob.ar/PDF/unidades%20tematicas/territorio%20y%20medio%20ambiente/eco%20regiones/Publicaciones/Cactus/nombre/gymnnigr.htm>
3) “Notes on the 1967 Seed List W.F.M.” The Cactus and Succulent Journal of Great Britain Vol. 29, No. 1 (FEBRUARY, 1967), pp. 16-18
4) Edward Anderson “The Cactus family” Timber Press, Incorporated, 2001
5) David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. "The New Cactus Lexicon" dh books, 2006
6) Charles, G. 2009. “Gymnocalycium in Habitat and Culture.” Graham Charles, Stamford.
7) Pilbeam, John. “Gymnocalycium: A Collector's Guide.” A. A. BALKEMA / ROTTERDAM / BROOKFIELD / 1995
8) Preston-Mafham Rod Preston-Mafham Ken “Cacti: The Illustrated Dictionary” 1995
9) Detlef Metzing “Nacktdistel und Spinnenkaktus Die Gattung Gymnocalycium.” Sonderausgabe der Deutsche Kakteen-Gesellschaft e.V., 2012, S. 99.
10) KuaS 2/2001, S. 37 f.
11) KuaS 4/2012.
12) Karteikarte 2012/07.

Cultivation and Propagation: Gymnocalycium nigriareolatumSN|15765]]SN|15765]] is a summer grower species that is easy to cultivate, but it grows rather slowly.
Growth rate: It is a relatively rapidly growing and easily flowering species that will make clumps given the best conditions.
Soils: It likes very porous standard cactus mix soil. Prefer a low pH compost, avoid substrata rich in limestone; otherwise growth will stop altogether.
Repotting: This plant needs plenty of space for its roots, repotting should be done every other year or when the it has outgrown its pot. Use pot with good drainage.
Watering: Needs moderate to copious waterings in summer, but do not overwater (Rot prone), keep dry in winter at a minimum temperature of 0°C.
Fertilization: Feed with a high potassium fertilizer in summer.
Hardiness: Reputedly resistant to frost if kept on the dry side prior to, and during, cold weather (hardy to -5° C, or less for short periods).
Exposition: The plant tolerates extremely bright situations but enjoys filtered sunlight or afternoon shade, inside it needs bright light, and some direct sun. Tends to bronze in strong light, which encourages flowering and heavy spine production, but is likely to suffer from sun scorch or stunted growth if over exposed to direct sunlight during the hottest part of the day in summer.
Uses: It is an excellent plant for container growing. It always looks good and stays small. It look fine in a cold greenhouse and frame or outdoor in a rockery.
Propagation: Division, direct sow after last frost. Seeds germinate in 7-14 days at 21-27° C in spring, remove gradually the glass cover as soon the plants will be well rooted (ca 1-2 weeks) and keep ventilated, no full sun for young plants! To make a cutting twist off a branch and permit it to dry out a couple of weeks, lay it on the soil and insert the stem end partially into the soil. Try to keep the cutting somewhat upright so that the roots are able to grow downward.