Origin and Habitat: Argentina (San Luis)

• Gymnocalycium megalothelon (Sencke ex K.Schum.) Britton & Rose
• Echinocactus megalothelos Sencke

• Gymnocalycium denudatum (Link & Otto) Pfeiff. ex Mittler
• Cereus denudatus (Link & Otto) Pfeiff.
• Echinocactus denudatus Link & Otto
• Echinocactus denudatus var. leheideliana F.Haage
• Echinocactus intermedius (Hildm.) Schelle
• Gymnocalycium denudatum var. octogonum (K.Schum.) Y.Itô
• Echinocactus denudatus var. octogonus K.Schum.
• Echinocactus denudatus f. octogonus (K.Schum.) Schelle
• Gymnocalycium intermedium (Hildm.) Y.Itô
• Echinocactus denudatus var. intermedius Hildm.
• Echinocactus intermedium (Hildm.) K.Schum.
• Gymnocalycium denudatum var. intermedium auct.
• Gymnocalycium megalothelon (Sencke ex K.Schum.) Britton & Rose
• Echinocactus megalothelos Sencke

• Gymnocalycium denudatum subs. angelae (Mereg.) Prestlé
• Gymnocalycium angelae Mereg.

• Gymnocalycium denudatum var. flavispinum (Mundt in Schelle) Y.Itô
• Echinocactus denudatus var. flavispinus (Mundt in Schelle) Y.Itô

• Gymnocalycium denudatum cv. Jan Suba hort.
• Gymnocalycium denudatum var. subai

Description: Gymnocalycium megalothelonSN|7888]]SN|1724]] is a form of Gymnocalycium denudatumSN|7888]]SN|7888]], but not sufficiently differentiated and considered merely a local variant of the latter. The key difference by which the two were separated, namely the presence of more prominent tubercles, yellow spines and white double flowers, seems entirely spurious, but plants from this population do differ in various respects from that on the common Gymnocalycium denudatumSN|7888]]SN|7888]].
Habit: It is a (usually) solitary geophytic cactus with five ribbed appearance.
Stem: Flattened or globular, grey-green to olive green, 5-6 cm in diameter, up to 5 cm tall, but in cultivation it can become 7-8 cm high (or more) over the years.
Ribs: 5-7, rounded, grooved transversely. Tubercles few per ribs, more prominent and "chinned" than in standard Gymnocalycium denudatumSN|1724]]SN|7888]].
Radial spines: Stout 5-7 up to 12, but usually less in young plants, flattened against the body.
Central spines: Usually absent, or 1 erected one.
Flowers: Are some of the largest in this genus, often growing larger than the plant!, 6 up to 7 cm long and wide, pure white and double petaled. Self-fertile!
Fruits: Green.

Bibliography: Major references and further lectures
1) David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. "The New Cactus Lexicon" dh books, 2006
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

Cultivation and Propagation: Gymnocalycium megalothelonSN|1724]]SN|1724]] 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 light frost if kept on the dry side prior to, and during, cold weather (hardy to -2 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: 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!