Astrophytum is a genus that now includes six North American cacti species found in Mexico and some areas of the U.S. Three of the six species are fairly common in residential cultivation, and they are sometimes known collectively as star cactus—named for the ribbed structure that has a star-like shape when the plant is viewed from above. The body is covered with aureoles from which sharp black or golden spines protrude. Several of the species grow white flocking on the body—an adaptation designed to protect the plant's tissues from the sunlight.
Astrophytum cacti are known for their beautiful growth habit and unusual shape (they are sometimes known as "living rocks") as well as for lovely yellow flowers that extend up from the apex.
Astrophytum species, like most cacti, are generally very slow-growing. Although mature specimens are quite impressive, it can take years to nurture an Astrophytum to that level. These are also very difficult plants to cultivate; they are often grafted onto an Echinopsis cactus for best results. Although not easy plants to nurture, if properly grown, these are among the most beautiful of North America cacti.
|Botanical Name||Astrophytum spp.|
|Common Names||Star cactus (individual species are known by other common names)|
|Mature Size||2 inches to 6 feet (varies by species)|
|Sun Exposure||Full sun|
|Soil Type||Coarse, sandy soil; cactus potting mix|
|Soil pH||7.0 to 8.0 (neutral to slightly alkaline)|
|Bloom Time||Varies by species|
|Hardiness Zones||9 to 11 (USDA)|
|Native Area||Mexico, portions of Southwest U.S.|
How to Grow Astrophytum Cacti
The needs of an Astrophytum cactus are the same as for other cacti—coarse, porous soil, and plenty of sunlight. While this cactus can be grown as a landscape garden specimen in the proper climate, it more commonly is used as a potted specimen for indoor growing or on decks and patios.
Astrophytums are not vigorous plants, and they require a lot of patience to reach their full potential. Keep regularly watering and feeding them during the growing season and repot annually to give them space to develop. They need a temperate, dry winter; make sure not to overwater them, which can cause root rot. If given sufficient care, they will develop marvelous flowers once they reach maturity.
Give Astrophytum cacti plenty of sunlight—at least 6 hours per day if growing outdoors. As a potted indoor plant, it can do fine in a window that gets direct sunlight or intense indirect light.
Like most cacti, these species need well-drained, fairly sandy soil and will likely perish in rich soils that retain moisture.
Despite their reputation as desert plants, many cacti, including the Astrophytum species, perform best if the soil gets a good drenching between periods where it dries out entirely. A potted plant should get a decent watering once a month during the growing season, but make sure the plant's roots don't sit in water. During the winter, watering can be less frequent.
Temperature and Humidity
These cacti like warm temperatures above 70 degrees Fahrenheit all summer, then slightly cooler, more temperate winter conditions. High humidity is not advised for these plants.
Fertilize during the growing season with a balanced, diluted fertilizer, such as 20-20-20, mixed into the water.
Propagating Astrophytum Cacti
Astrophytum cacti propagate by seeds, which can be collected from the dried flowers. These seeds are fairly fragile: Be gentle with them before planting in a tray or pot filled with cactus potting mix. Just barely cover the seeds with potting medium.
The seeds have a fairly short shelf life and need to be planted quickly after harvesting to have a reasonable chance at rooting. Seal the tray or pot with a plastic cover to help contain moisture and heat. It can help to soak seeds before they’re planted.
Once the seedlings appear, uncover the tray or pot and gradually acclimate them with increasing periods spent in direct sunlight. Within a few weeks, the seedlings can be transplanted into their permanent containers filled with cactus potting mix.
Potting and Repotting
Repot these cacti regularly to help them develop. Astrophytums should be repotted at the beginning of the growing season to allow them to grow into the impressive specimens for which the genus is known. Wearing sturdy gloves, lift the plant out all at once, then move to a larger pot and backfill with potting mix. Don’t overwater or overfeed newly repotted cacti, as disturbance can be hard on them.
Varieties of Astrophytum Cacti
- Astrophytum myriostigma (commonly called bishop's cap cactus, bishop's hat or bishop's miter cactus) has the star-shape structure common to the genus, which in this case takes the domed-like appearance of a bishop's headwear. It has three to seven well-defined ribs, and more may appear as the plant ages, giving it a more cylindrical appearance. This plant can exceed 3 feet in height and 8 inches in diameter when it grows in the garden, though as a houseplant it is generally under 1 foot in height. The flowers are creamy yellow with an orange or red base. The ‘Super Kabuto’ strain is highly prized.
- Astrophytum capricorne (commonly called goat's horn cactus) grows about 10 inches tall and 4 inches across. It has a gray-green color with prominent ribs and yellow flowers.
- Astrophytum ornatum (bishop's cap or monk's hood cactus) is the tallest member of the genus, growing as much as 6 feet in height and 12 inches in width. It flowers with yellow blooms throughout the summer. This is the fastest-growing species in the genus.
- Astrophytum asterias (known as sand dollar cactus, sea-urchin cactus, star cactus, or star peyote) is a short, stubby cactus that reaches only about 2 1/2 inches high with a 6-inch diameter. This is a rare, hard-to-grow species not often seen in residential use.
- Astrophytum caput-medusae is a rare, endangered species that does not have the same star-shaped structure as other members of the genus. Instead, it has narrow, cylindrical stems up to 7 inches long, with yellow flowers. This species is rarely grown as a residential plant.
- Astrophytum coahuilense is another rare species not often grown in cultivation. It is very similar in appearance to A. myriostigma, with five ribs and a similar size, but its body has a softer gray color.