Origin and Habitat: La Rioja and Catamarca, Argentina
Altitude: 1200-2000 metres above sea level.
Habitat: It grows in various environs and elevations in hilly terrain and steep slopes, among stones, grasses and bushes.

• Gymnocalycium weissianum Backeb. in Backeb. & F.M.Knuth

• Gymnocalycium hossei A.Berger
• Echinocactus hossei F.Haage ex A.Berger
• Gymnocalycium guanchinense Schütz
• Gymnocalycium hossei var. breviflorum (Backeb.) H.Till
• Echinocactus mazanensis var. breviflorus Backeb.
• Gymnocalycium hossei var. crassispinum Y.Itô
• Gymnocalycium hossei var. nidulans (Backeb.) H.Till
• Gymnocalycium nidulans Backeb. in Backeb. & F.M.Knuth
• Gymnocalycium mazanense (Backeb.) Backeb.
• Echinocactus mazanensis Backeb.
• Gymnocalycium mazanense var. ferox Backeb.
• Gymnocalycium rhodantherum (Boed.) H.Till
• Echinocactus rhodantherus Boed.
• Gymnocalycium mazanense var. rhodantherum (Boed.) Backeb.
• Gymnocalycium rhodantherum var. cinerascens H.Till
• Gymnocalycium rhodantherum subs. recii (Boed.) Milt
• Gymnocalycium weissianum Backeb. in Backeb. & F.M.Knuth
• Gymnocalycium weissianum var. atroroseum Backeb. in Backeb. & F.M.Knuth

Description: Gymnocalycium weissianumSN|14401]]SN|14401]] is one of the several forms of the very variable Gymnocalycium hosseiSN|14401]]SN|14387]] which form a complex of very closely related taxon. It is still unclear whether this variable complex is one or several species. There is so much variation even in the same population that most authors choose to lump them together. Gymnocalycium weissianumSN|14387]]SN|14401]] is quite variable and distinguishes for the covering of stout bright greyish-white interlacing spines.
Habit: It is a geophyte, stem succulent plant usually solitary and barely rising above ground level with a swollen underground root system.
Stem: Globular, somewhat flattened and slightly depressed at the centre (or slightly elongated in cultivation), dark grey-green, olive-green, dark chocolate brown or red-violet, up to 10 cm in diameter.
Ribs: Gymnocalycium weissianumSN|14401]]SN|14401]] has about 19 ribs, straight, broad, flat, divided into angular warts with prominent angular chins below the the large woolly areoles , with narrow furrows between them.
Areoles: Oval 10-15 mm apart, slightly furnished with yellowish grey wool.
Central spine: Occasionally one central, brown, becoming grey with age up to 4 cm long.
Radial spines: 6-8, pinkish-brown when still growing, but turning soon bright, greyish-white ,spreading, curving backward encasing the brown body, one pointing downward, others to the sides, up to 20 (or more) mm long, but quite variable in length.
Flowers: Near the apex, 70 mm in length and 60 mm in diameter whitish to reddish pink, sometimes with brownish tint, with short floral tubes.
Flowering time: Blooms appear recurrently from June to autumn and flowers well as a young plant.

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) 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
5) Curt Backeberg “Die Cactaceae: Handbuch der Kakteenkunde” Gustav Fischer Verlag, Stuttgart New York 1982–1985
6) John Borg “Cacti: a gardener's handbook for their identification and cultivation” Blandford P., 1970
7) Clive Innes, Charles Glass "Cacti" Portland House, 01/May/1991

Cultivation and Propagation: Gymnocalycium weissianumSN|14401]]SN|14401]] 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.