Acanthocalycium thionanthum (Speg.) Backeb.
Acanthocalycium thionanthum subsp. thionanthum, Echinocactus thionanthus, Echinopsis thionantha, Lobivia thionantha
Acanthocalycium thionanthum, also known as Echinopsis thionantha, is a small cactus with spherical to cylindrical stems with 9 to 25 ribs lined with clusters of spines. The stems can grow up to 26 inches (65 cm) tall and 10 inches (25 cm) in diameter and are usually solitary or with a few basal offshoots. The stem color ranges from green to grey-green. The spine-bearing areoles are large and circular, with yellow to brown felt when young but become naked as they age. The spines are up to 1.2 inches (3 cm) long, awl-like, and at first black or light brown, becoming grey or whitish-yellow as they age. Each areole bears 5 to 10 radial and 1 to 4 central spines.
The plant blooms in late spring and early summer, and its flowers range from yellow to red, including all the intermediate tones. The flowers are funnel-shaped and up to 2 inches (5 cm) long, with the floral tube covered in brownish-white bristles and spine-tipped scales.
Acanthocalycium thionanthum is native to Argentina. It grows on rocky mountain slopes in Salta, Tucumán, Catamarca, and La Rioja provinces.
The specific epithet "thionanthum (thee-oh-NAN-thum) means "sulfur yellow flower" and refers to the pale yellow flowers of the type specimens.
How to Grow and Care for Acanthocalycium thionanthum
Light: Acanthocalycium thionanthum thrives when exposed to intense sunlight and can be grown in full sun to partial shade. Placing it near a window that receives about 6 hours of direct sun daily is ideal if grown indoors. It is best to move it outdoors in the spring but gradually expose it to direct sunlight to avoid sunburn.
Soil: To promote healthy plant growth, it is important to use sandy or gritty soil with good drainage. Overwatering and poor drainage can cause root rot. A commercial potting mix for cacti will work well, or you can make your own.
Temperature: This cactus can withstand high temperatures, but during the winter, it prefers cooler temperatures. It grows best in USDA Plant Hardiness Zones 9a to 11b, with average minimum winter temperatures ranging from 20 to 50 °F (-6.7 to 10 °C).
Watering: During the growing season, water Acanthocalycium thionanthum thoroughly but allow the soil completely dry out before watering again. Avoid letting the container sit in a saucer of water. During the winter, suspend watering.
Fertilizing: Feeding this cactus with a water-soluble fertilizer is recommended for optimal growth. However, during the winter, when it goes dormant, fertilization is not required.
Repotting: Acanthocalycium thionanthum can stay in its pot for years. Repot it only when it has outgrown its current pot or when the soil has degraded, ideally in late winter or early spring. Before repotting, make sure the soil is dry. Choose a slightly larger pot with drainage holes at the bottom.
Propagation: Since it rarely produces offsets, this cactus is usually propagated by seeds. Sow the seeds in late spring or summer.
Learn more at How to Grow and Care for Echinopsis.
Toxicity of Acanthocalycium thionanthum
Acanthocalycium thionanthum is non-toxic to humans or pets. However, keep it away from pets and children as it has sharp spines.
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