Origin and Habitat: WO66, Taninga, Cordoba, Argentina 1000m 

• Gymnocalycium knollii Mereg.

• Gymnocalycium bodenbenderianum (Hosseus ex A.Berger) A.W.Hill
• Echinocactus bodenbenderianus Hosseus ex A.Berger
• Gymnocalycium pseudoragonesei Schuetz
• Gymnocalycium riojense Frič ex H.Till & W.Till
• Gymnocalycium riojense subs. guasayanense (Strigl) H.Till
• Gymnocalycium riojense var. guasayanense Strigl
• Gymnocalycium riojense var. guthianum H.Till & W.Till
• Gymnocalycium riojense var. mirandaense H.Till & W.Till
• Gymnocalycium riojense subs. paucispinum Backeb. ex H.Till & W.Till
• Gymnocalycium paucispinum (Backeb. ex H.Till & W.Till) H.Till
• Gymnocalycium riojense subs. piltziorum Schütz ex H.Till & W.Till
• Gymnocalycium riojense var. pipanacoense H.Till & W.Till
• Gymnocalycium riojense var. platygonum Schütz ex H.Till & W.Till
• Gymnocalycium platygonum (H.Till & W.Till) Pilbeam
• Gymnocalycium riojense var. sanjuanense H.Till & W.Till
• Gymnocalycium riojense var. vertongenii Amerh.
• Gymnocalycium triacanthum Backeb.

• Gymnocalycium bodenbenderianum subs. intertextum (Backeb. ex H.Till) H.Till
• Gymnocalycium intertextum Backeb. ex H.Till
• Gymnocalycium bodenbenderianum subs. intertextum var. moserianum Schuetz ex H.Till
• Gymnocalycium intertextum var. moserianum H.Till
• Gymnocalycium moserianum Schütz
• Gymnocalycium knollii Mereg.
• Gymnocalycium kozelskyanum Schütz
• Gymnocalycium riojense subs. kozelskyanum Schuetz ex H.Till & W.Till

Description: This plant has been introduced in Europe with the field number of Wilhelm Knoll WO 66 and never validly described. It is related with Gymnocalycium moserianum and Gymnocalycium intertextum [Gymnocalycium intertextum var. moserianum forma?].
Spines: 3(-5) up to 5 cm long brownish to blackish turning grey as they ages.
Root: Tap root.

Bibliography: Major references and further lectures
1) Edward Anderson “The Cactus family” Timber Press, Incorporated, 2001
2) 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
3) David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. "The New Cactus Lexicon" dh books, 2006
4)Curt Backeberg “Die Cactaceae: Handbuch der Kakteenkunde” Gustav Fischer Verlag, Stuttgart New York 1982–1985

Cultivation and Propagation: Gymnocalycium knollii is a summer grower species that is easy to cultivate.
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 -5 C ° 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.
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: 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!