Origin and Habitat: Corrientes, Mercedes, Argentina. Gymnocalycium mesopotamicumSN|15257]]SN|15257]] has been reported from a very small area and even though it might be found in surrounding areas, the area of potential habitat is still small (extent of occurrence 200-500 km2). The total number of plants in the largest population is probably less than 1,000. There are, however, many more hills in the area where the species could grow but they haven't been explored, so the population could be much larger than estimated.
Altitude range: About 200 metres above sea level.
Habitat and Ecology: This cactus grows somewhat concealed in grassy places with stones where the soil is shallow. There is grazing in the area where this species occurs, however, it is not intensive as it is impossible to grow artificial grassland because the area has many rocky outcrops. Grazing even favours the growth of these plants as it buffers competition for light. The only damage that can be caused by grazing animals is trampling on the plants. Another threat is quarrying, this has happened in the past and one locality was severely affected, however more plants were found in the surrounding grasslands and no more quarrying has taken place.

• Gymnocalycium mesopotamicum R.Kiesling

Description: Gymnocalycium mesopotamicus is a small cactus often solitary in habitat, but freely branching in cultivation where it makes pleasing clumps.
Stem: Flattened-spherical with an umbilicate depression at the apex, dark green, 1,2-2,5 cm tall and up to 4 cm in diameter (but often more in cultivation)
Ribs: 7-9, low and obtuse, well developed or rather indistinct, rounded, with transverse grooves.
Areoles: Wide, oval-elongated, slightly sunken at the top of the tubercle.
Central spines: Absent.
Radial spines: 9-12, unequal, arising from the side on the lower part of the areole, 2-9 mm long, all bristle-like, flexible, adpressed, reddish brown at first, later greyish or pink-tinged.
Blooming Habits: Flower pure white up to 5,5 cm long and 6,5 cm in diameter, outer perianth segments pinkish with darker brownish mid-stripe, inner ones pure white lanceolate, throat reddish, calyx bare and scaly.
Fruit: Cub-shaped to slender-clavate, dull green, 20-30 mm long, 7-8 mm in diameter.
Seeds: 1,7-1,8 mm diameter, dark brown to black, with conspicuous hilum.

Bibliography: Major references and further lectures
1) Cactus & Succulent Journal of Great Britain 42: 39. 40 (1980)
2) Kiesling, R. “Gymnocalycium mesopotamicum sp. Nov.” Cactus and Succulent Journal (Los Angeles) 42 : 39-42. 1980
3) Edward Anderson “The Cactus family” Timber Press, Incorporated, 2001
4) James Cullen, Sabina G. Knees, H. Suzanne Cubey "The European Garden Flora Flowering Plants: A Manual for the Identification of Plants Cultivated in Europe, Both Out-of-Doors and Under Glass" Cambridge University Press, 11/Aug/2011
5) David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. "The New Cactus Lexicon" dh books, 2006
6) Kiesling, R. 2013. Gymnocalycium mesopotamicum. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 11 May 2015.

Cultivation and Propagation: Gymnocalycium mesopotamicus is a summer grower species that is easy to cultivate.
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 -12 C ° C, or less for short periods), but for safe cultivation it is best to avoid freezing temperatures.
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.
Pests & diseases: It may be attractive to a variety of insects, but plants in good condition should be nearly pest-free, particularly if they are grown in a mineral potting-mix, with good exposure and ventilation. Nonetheless, there are several pests to watch for:
- Red spiders: Red spiders may be effectively rubbed up by watering the plants from above.
- Mealy bugs: Mealy bugs occasionally develop aerial into the new growth among the wool with disfiguring results, but the worst types develop underground on the roots and are invisible except by their effects.
- Scales: Scales are rarely a problem.
- Rot: This species is particularly easy and accommodating, seldom suffer of cryptogamic diseases. Rot it is only a minor problem with gymnocalyciums if the plants are watered and “aired” correctly. If they are not, fungicides won't help all that much.
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.