Acanthocalycium thionanthum is native to Argentina Northwest where the plant grows on mountain slopes and in rocky soils. The plant can spread up to 3000 m of altitude.
Acanthocalycium thionanthum is a spherical cactus belonging to the Cactaceae botanical family. The plant has a solitary habit but in age can branch very slowly, it can reach up to 12 cm in height and 10 cm in diameter. The stem is globose to spherical, light green to dark blue-green in color, arranged in 9-15 well marked ribs. The areoles are white and woolly and bear the spines. The spines are long, sharp, spider-like, black at first and yellowish in age. Each areola bears 5-10 radial spines and 1-4 central spines, but number, shape and size vary by populations. Blooming occurs from the early Spring throughout the Summer and the blossoms are borne near the apex of the stem. Flowering is possible only in individuals 3 years old; in this age these plant reach the maturity. The flowers are bell-shaped, bright yellow and rarely white or orange. The floral tube has many bristle-like, tipped, brown scales that give the name to the genus.
This is a fast growing plant, easy to cultivate. The plant needs a direct sun-light exposure all the year, it encourages flowering and heavy spine production. Temperatures below 8 ° C can damage the plant so it is best to shelter it or place it in a cold greenhouse during the winter. Too low temperatures can cause the stem or leaves to break due to water freezing inside the tissues. Temperatures between 10 and 15 °C allow the plants to enter vegetative rest which is essential for the flowering of the following year. Plants should not be placed inside the house where average temperatures of 20 degrees prevent vegetative rest. The perfect soil is a well-drained soil that let the water to drain away and avoid root rot. To achieve this feature, you can mix the pumice, clay and loam. The pumice should always be placed on the bottom of the pot. Remember to use a perforating pot to drain excess water. Watering can be done regularly during the vegetative period. During the vegetative period you can water the plant every 7 days with half a glass of water, checking that the soil is completely dry before watering again; in winter you should stop the watering to allow the plant to enter dormancy. Decrease the amount of water if the plant is kept indoors or if the pot is smaller than 12 cm. The plant needs to be fed a high-potassium fertilizer in the summer, but it no longer needs it throughout the year. You can dilute the fertilizer once a month in the irrigation water. If the pot starts to be too small for the plant you can repot the plant in a pot 2 cm wider. Repotting should be done early in the growing season with fresh new potting soil; when the plant reaches 12 cm in diameter it no more needs it. Be careful to red spiders and mealy bugs.
Propagation is usually done by seed but cutting is also possible. By cutting you can use the offsets during the spring. Cut an offset and then let it dry; after a few days the cut surface will dry and a callus will form, then place the cutting in a mixture of sand, soil and pumice. To increase the success of propagation you can make two or more cuttings at the same time. It is advisable to use rooting hormone at the base of the cut to energize root development. For cuttings it is recommended temperatures around 20 °C. By seed it is very simple to propagate the plant, it is enough to sow the seed in a sandy loam and keep it with a high level of -humidity and at temperature of 14 C°. At 21-27 °C the seeds germinate in 7-14 days; after the plant rooted keep them away from direct sun-light in a ventilated place.
The name of this family of plants means, literally, “chalice with thorns” (from Greek ákantha, thorns, and cályx, chalice). The slivers on the floral tube and on the ovary are in fact changed in thorns. The specific epithet “thionanthum” in Greek means ‘sulphurous yellow flowers’ and refers to the bright yellow flowers.
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