Description
Ariocarpus retususus commonly known as the “Star Rock” is one of the largest species that distinguishes for the fat triangular tubercles forming a starry rosette. It is a widespread and extremely variable plant. Tubercle size and shape vary widely, a terminal areole is sometimes present at the tip.
Crested form: The crested Star Rock (Ariocarpus retusus f. cristata) is a very rare, slow-growing and highly priced crested cactus forming odd sculptural shapes. Its stems fan out and can get very big making a spectacular specimen. The crested form - despite to its beauty - is still very rare, pricey and sought after by collectors.
Habit: Solitary, slow growing geophyte cactus with tubercles slightly projecting above ground level. The tuberous body is mostly below the soil.
Stem: Grey, or blue-green, fan shaped that will slowly forms brain-shaped cushion that can reach a considerable size (up to 3-12 cm high and 70 cm in diameter, or more).
Tubercles: Leaf-like, divergent, erect, basally compressed, usually becoming attenuate at the apices, convex or nearly flattened adaxially and often with shallow adaxial undulations or wrinkling, not fissured, 1,5-4 cm long, 1-3,5 cm wide, nearly as wide as long.
Areoles: At the tips of the tubercles, rounded, 1-5 mm in diameter often only on younger specimens.
Flowers: Diurnal 4-5cm in diameter, 2-4,2 cm long, white to pink (or magenta), occasionally with reddish midribs.
Roots: Tap root.
Flowering time: Autumn.
Fruits: White, green, or rarely pinkish, 10-25 mm long, 3-10 mm in diameter.
Remarks: All of the Ariocarpus seedlings are spiny and quite different from adult specimens, though the spines are feather-like.
Origin and Habitat
The natural species is widely distributed in Mesico from north of Saltillo, Coahuila southwards to San Luis Potosi. Also found in Tamaulipas, Zacatecas and Nuevo Leon.
Cultivation and Propagation
It is not too difficult in a greenhouse, although grows quite slowly. It is usually seen as a grafted plant but can grow on its own roots too.
Soil: Use a mineral well permeable soil with little organic matter (peat, humus).
Exposure: They need a good amount of light shade to full sun this help to keep the plants healthy, although slow growth.
Watering: Water sparingly from March till October (weekly during summertime, if the weather is sunny enough), with a little fertilizer added. Less or no water during cold winter months, or when night temperatures remain below 10° to prevent root loss. It is sensitive to overwatering (rot prone).
Fertilization: Feeding may not be necessary at all if the compost is fresh then, feed in summer only if the plant hasn’t been repotted recently. Do not feed the plants from September onwards as this can cause lush growth which can be fatal during the darker cold months.
Hardiness: Keep perfectly dry in winter at temperatures from 5 to 15 degrees centigrade. (but it is relatively cold resistant and hardy to -5° C, or possibly colder for short periods) In the rest period no high atmospheric humidity!! (Temperature Zone: USDA 9-11)
Crested growth: Unlike ‘monstrose’ varieties of plants, where the variation from normal growth is due to genetic mutation, crested growth can occur on normal plants. Sometimes it’s due to variances in light intensity, or damage, but generally the causes are unknown. A crested plant may have some areas growing normally, and a cresting plant that looks like a brain, may revert to normal growth for no apparent reason. If you have any of the crested part left you need to remove the normal growth and leave the crested part behind this will need to be done regularly.
Propagation: Grafting or cuttings. Plants are usually grafted onto column-shaped cacti but proved to be able to produce their own roots if degrafted. Cuttings will take root in a minimum temperature of 20° C (but better in hot weather). Cuttings of healthy shoots can be taken in the spring and summer. Cut the stem with a sharp, sterile knife, leave the cutting in a warm, dry place for a week or weeks (depending on how thick the cutting is) until a callus forms over the wound. Once the callus forms, the cutting may be inserted in a container filled with firmed cactus potting mix topped with a surface layer of coarse grit. They should be placed in the coarse grit only; this prevents the cut end from becoming too wet and allows the roots to penetrate the rich compost underneath. The cuttings should root in 2 to 6 weeks. Large crested piece must be placed on the soil surface without burying the plant base down in the soil.
Bibliography
Major references and further lectures
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Edward F. Anderson “The Cactus Family” Timber Press, 2001
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Curt Backeberg “Cactus Lexicon” Sterling Publishing Company, Incorporated, 1978
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Hiroshi Hirao “Colour encyclopaedia of cacti” Japan 1979 (Japanese language and script)
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Willy Cullmann, Erich Götz (Dozent Dr.), Gerhard Gröner “The encyclopedia of cacti” Portland, OR: Timber Press, 1986
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David Hunt, Nigel Taylor “The New Cactus Lexicon” DH Books, 2006
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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
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Fitz Maurice, B & Fitz Maurice, W.A. 2013. “Ariocarpus retusus.” The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 12 December 2014.
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