Origin and Habitat: Tarija, Chuquisaca, Santa Cruz, Bolivia
Altitude: 900-2500 metres above sea level.
Habitat: It grows often in deep, rich soil, rather in sunny rocky areas, on slopes or under spiny shrubs in the drier parts of the Inter-andean valleys (Rio Grande drainage system), dominated by a columnar cactus forests, known as cardonales. Cacti commonly represented in this forests include Cleistocactus parviflorusSN|3762]]SN|7302]], Cleistocactus samaipatanusSN|7302]]SN|3762]], Corryocactus pulquinensisSN|9834]]SN|9834]]. Harrisia tetracanthaSN|8739]]SN|8739]], Neoraimondia herzogiana, Opuntia cochabamhensis, Pereskia diaz-romeroana. and Pereskia weberiana, Lepismium ianthotheleSN|8143]]SN|8143]], Samaipaticereus corroanus, Yungasocereus inquisivensisSN|2995]]SN|2995]], Echinopsis caineanaSN|21037]]SN|10698]], Echinopsis comarapana, Echinopsis obrepandaSN|10698]]SN|21037]], Parodia mairananaSN|5775]]SN|5775]], and Espostoa guentheriSN|6818]]SN|6818]].

• Gymnocalycium pflanzii var. riograndense (Cárdenas) Donald
• Gymnocalycium riograndense Cárdenas
• Gymnocalycium zegarrae subs. riograndense (Cárdenas) H.Till & Amerh.
• Gymnocalycium zegarrae var. riograndense (Cárdenas) H.Till & Amerh.

• Gymnocalycium pflanzii (Vaupel) Werderm.
• Echinocactus pflanzii Vaupel
• Gymnocalycium comarapense Backeb.
• Gymnocalycium lagunillasense var. karmiri Kníže in Pilbeam
• Gymnocalycium marquezii Cárdenas
• Gymnocalycium marquezii var. angostura hort.
• Gymnocalycium marquezii var. formesense hort.
• Gymnocalycium marquezii var. pilcomayo hort.
• Gymnocalycium pflanzii var. albipulpa F.Ritter
• Gymnocalycium pflanzii subs. argentinense H.Till & W.Till
• Gymnocalycium marquezii var. argentinense Backeb.
• Gymnocalycium pflanzii f. chuquisacanum (Cárdenas) Donald
• Gymnocalycium chuquisacanum Cárdenas
• Gymnocalycium pflanzii var. izozogsii (Cárdenas) Donald
• Gymnocalycium izozogsii Cárdenas
• Gymnocalycium pflanzii var. lagunillasense (Cárdenas) Donald
• Gymnocalycium lagunillasense Cárdenas
• Gymnocalycium pflanzii var. millaresii (Cárdenas) Donald
• Gymnocalycium millaresii Cárdenas
• Gymnocalycium zegarrae subs. millaresii (Cárdenas) H.Till & W.Till
• Gymnocalycium pflanzii var. riograndense (Cárdenas) Donald
• Gymnocalycium riograndense Cárdenas
• Gymnocalycium zegarrae subs. riograndense (Cárdenas) H.Till & Amerh.
• Gymnocalycium zegarrae var. riograndense (Cárdenas) H.Till & Amerh.
• Gymnocalycium zegarrae var. coquimbana hort.
• Gymnocalycium zegarrae var. saipinense hort., Kníže

• Gymnocalycium pflanzii subs. zegarrae (Cárdenas) G.J.Charles
• Gymnocalycium pflanzii var. zegarrae (Cárdenas) Donald
• Gymnocalycium zegarrae Cárdenas

Description: Gymnocalycium pflanzii var. riograndenseSN|18750]]SN|18755]] is a single headed, flattened globular cactus, with peculiar spine clusters characteristically bent backward in a distinctive fashion and with short tubed flowers with bluish-red centres. This species is characterized by a marked variability in the length and shape of the spines that resemble the long leg of a spider and is hard to tell it apart from the other similar Gymnocalycium pflanziiSN|18750]]SN|18750]] forms and varieties.
Stem: Flattened-globose, up to 20 cm in diameter, 10 cm tall pale green, to olive green that can tinge of orangish purple in full sun. The epidermis has a velvety appearance. The apex is slightly depressed and woolly.
Ribs: 10 to 12 thick, large, divided into polygonal rounded (not chinned) tubercles, separated by arched furrows.
Roots: Fibrous
Areoles: Large long, oval 10x14 mm, with cream-yellowish wool, then blackish dismantling with time.
Spines: Strong, very stiff, bent backward, initially blackish with a brown base then becoming grey-pinkish to whitish brown with black point.
Radial spines: (5-)7-9 up to 25 mm long
Central spines:One (or two) of the same length.
Flowers: Infundibuliform near the apex, diameter and height 45-50mm. Pericarpell dull green very short. External tepals spatulate, slightly bent outward , white with brownish green midrib. Inner tepals intern narrower, spatulate and slightly denticulate, white with carmine/pink/bluish/purplish throat. Style carmine with carmine pink 10-12 lobes. Pollen yellow.
Fruits: Globose of approximately 2 cm of diameter, becoming red carmine with maturity. Pulp intense cherry-red.
Seeds: Microsemineum type, section pirisemineum, very small (0.6 X 0.4 mm), testa reddish shining and smooth.
Remarks: Gymnocalycium pflanziiSN|18755]]SN|18750]] is a very variable species with a wide distribution and several controversial varieties, forms and subspecies has been described and it has received many names.

Bibliography: Major references and further lectures
1) Sara Oldfield “Cactus and succulent plants: status survey and conservation action plan” IUCN, 1997
2) Edward Anderson “The Cactus family” Timber Press, Incorporated, 2001
3) 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
4) David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. "The New Cactus Lexicon" dh books, 2006
5) N. L. Britton, J. N. Rose “The Cactaceae. Descriptions and Illustrations of Plants of the Cactus Family.” Volume 4, The Carnegie Institution of Washington, Washington 1923
6) Curt Backeberg “Die Cactaceae: Handbuch der Kakteenkunde” Gustav Fischer Verlag, Stuttgart New York 1982–1985

Cultivation and Propagation: Gymnocalycium are very gratifying plants, their culture is easy and their flowering is abundant if we give them a well drained relatively rich substrate (if possible not limestone).
Growth rate: It is a relatively rapidly growing and easily flowering species.
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: Usually by Seeds.. 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! The grafting is useless (excluded the cultivars deprived of chlorophyll), it does not bring anything concrete.