How to Grow and Care for Star Cactus

astrophytum cacti in pots

The Spruce / Kara Riley

The Astrophytum genus includes six North American cacti species found in Mexico and the southwest portion of the United States. Three of the six species are common residential cultivars, collectively known as "star cactus," and named for their ribbed structure that has a star-like shape. The body of a star cactus is covered with aureoles that contain sharp black or golden spines. A few species grow white flocking (an adaptation designed to protect the plant's tissues from sunlight) as well as yellow, orange, or red flowers that bloom from the plant's center.

Sometimes referred to as a "living rock," star cactus is very slow-growing, with mature specimens being quite impressive. It is also a very difficult plant to propagate, and often needs grafting onto an Echinopsis cactus for best results. Although star cactus requires patience, when properly grown, it is among the most beautiful of the North American cacti.

Common Name Star cactus
Botanical Name Astrophytum spp.
Family Cactaceae
Plant Type Cactus, perennial, succulent
Mature Size 2-72 in. tall, 2-12 in. wide
Sun Exposure Full
Soil Type Sandy, well-drained
Soil pH Alkaline
Bloom Time Summer, fall
Flower Color Yellow, orange, red
Hardiness Zones 9-11 (USDA)
Native Area North America

Star Cactus Care

Star cactus, similar to other cacti, needs coarse, porous soil and plenty of sunlight to flourish. While this cactus can be grown as a landscape garden specimen in the proper climate, it is more commonly seen potted indoors or as a decor piece on decks and patios. Star cactus requires patience, as this plant is a slow bloomer and may not reach its full potential for several years. Keep this cactus regularly watered and lightly fertilized during its growing season, and then repot it annually to allow more room for development, Star cactus can only be grown outside in climates with temperate, dry winters. When properly taken care of, this species of cactus will develop marvelous flowers once it matures.

closeup of astrophytum cacti
The Spruce / Kara Riley
closeup of astrophytum cacti
The Spruce / Kara Riley
varieties of astrophytum cacti
The Spruce / Kara Riley 
Astrophytum myriostigma
 brytta / Getty Images
astrophytum ornata
The Spruce / Kara Riley
astrophytum asterias
The Spruce / Kara Riley 

Light

Plant star cactus outdoors in an area that receives at least six hours of direct sunlight per day. Potted indoor cactuses need at least four hours of direct sunlight, which can be achieved by situating them on a sunny south or west-facing windowsill.

Soil

Like most cacti, this species requires well-drained, sandy, porous soil. Potting soils that contain perlite, gravel, and pebbles, along with organic material to retain some moisture, work best for indoor specimens. Outdoors, star cactus does well in a xeriscaped environment, or in rock gardens, as its native habitat consists of sparse vegetation and grasses. Star cactus prefers acidic soil with a pH between 5 and 6.5. Alkaline soil conditions may cause the plant to perish.

Water

Despite its reputation as a desert plant, many cacti, including star cactus, grow best when caretakers adhere to the "soak and dry" method. It's best to give the plant a good drenching, and then let it dry out entirely between waterings. Potted and outdoor plants should both get a deep soak once a month during the growing season, making sure drainage is complete so that the plant's roots don't sit in water. During periods of rain and during the winter, refrain from watering your outdoor cactus altogether.

Temperature and Humidity

Star cactus likes warm temperatures above 70 F all summer long, with slightly cooler, more temperate, winter conditions. Surprisingly, this plant can withstand short bursts of temperatures as low as 20 F. In prolonged periods of 90 F or higher, increase the watering schedule to keep the plant from succumbing to stress. Growing star cactus in areas with high humidity is not advised, as this plant may suffer root rot and other diseases when humid conditions persist.

Fertilizer

Most cacti thrive in poor soil conditions, but indoor star cactus can benefit from light fertilization during its growing period (June through September). Monthly feedings with a balanced houseplant fertilizer (one with a 20-20-20 ratio) diluted to half strength may increase vigor in your plant. Refrain from fertilizing your cactus in the late fall as it prepares for dormancy, and do not fertilize it at all over the winter.

Types of Star Cactus

Different varieties of star cacti yield different traits: some are short and some are tall, some feature a green color, while others are grayer. Most of the varieties flower, however, it may take years before blooming commences.

Below are some of the favorite cultivated and wild varieties:

  • Astrophytum myriostigma is commonly called bishop's cap cactus, bishop's hat, or bishop's miter cactus. This variety has a star-shaped structure and a dome-like appearance. It features three to seven well-defined ribs and can exceed 3 feet in height and 8 inches in diameter when grown in the garden. Indoors, expect this houseplant to top out at 1 foot in height. Bishop's cap grows creamy yellow flowers with an orange or red base.
  • Astrophytum capricorne (or goat's horn cactus) grows about 10 inches tall and 4 inches wide. 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 to 6 feet in height and 12 inches in width at maturity. This variety boasts yellow blooms throughout the summer and is the fastest-growing species in the genus.
  • Astrophytum asterias, also 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 an endangered species that does not have the same star-shaped structure as other members of the genus. Instead, this variety has narrow, cylindrical stems up to 7 inches long and bears yellow flowers. This species is rarely grown as a residential plant.
  • Astrophytum coahuilense is another rare species not often grown in cultivation. It is similar in appearance to Astrophytum myriostigma, with five ribs, but its body has a soft gray color.

Pruning

Due to its uni-bodied shape, star cactus never needs pruning, however, deadheading spent blooms will create an aesthetically pleasing specimen.

How to Grow Star Cactus From Seed

Astrophytum cacti propagate only by seed, which can be collected from the dried flowers. These seeds are fairly fragile, so be gentle with them while handling. The seeds have a fairly short shelf life and need to be planted quickly after harvesting to have a reasonable chance of rooting. Soaking the seeds overnight before they’re planted helps the process.

To grow star cactus from seed, fill a tray with a cactus potting mix and broadcast the seeds over the tray. Barely cover the seeds with potting medium and mist with water until the soil is moist. Seal the tray with a plastic cover to help retain moisture, and place it in a sunny window, maintaining a 70 F temperature and misting occasionally. Be patient, as it can take several weeks for seedlings to appear. Once they do, uncover the tray and gradually acclimate them with increasing periods of direct, outdoor sunlight. Within a few weeks, the seedlings can be transplanted into their permanent containers filled with a cactus potting mix.

Potting and Repotting

This type of cactus needs regular repotting to assure it reaches mature size. Astrophytum should be repotted at the beginning of each growing season in order to allow it to grow into the impressive specimen for which the genus is known. To do so, first, put on a pair of sturdy gloves and obtain a clay or terracotta pot with ample drain holes. Next, fill the new pot with a cactus potting mix, and then lift the plant out of the pot and move to a larger pot, backfilling it with potting mix. Water the new plant and allow it to drain completely. Then, wait a while before fertilizing, as the disturbance of repotting can be hard on a cactus.

Overwintering

Outdoor cacti require little care in the winter. Simply refrain from watering your star cactus during its dormancy period, starting up again only when new growth is apparent in the spring. Indoor cacti can be overwintered in a cool, dry room that receives some sunshine during the day. Water indoor specimens only once a month and refrain from fertilizing altogether.

Common Pests & Plant Diseases

Watch out for common cactus pests, like aphids and scale. Both can be identified by the presence of the bugs on the cactus body, and can often be taken care of by quarantining the plant and blasting it with water from the garden hose. You can also use an eco-friendly pesticide, or a soap and water mixture, to spray the plant. Then, simply wipe away bugs with a cloth.

Bacteria and fungus can also move into a neglected cactus when broken leaves and lesions are open to infection. If a fungus or bacteria moves in, you may see soft spots on your plant. Treat lesions with a mixture of baking soda, hydrogen peroxide, and liquid soap dissolved in water and sprayed onto the plant. However, if the infection is severe, treatment may not work.

How to Get Star Cactus to Bloom

Star cactus does not start blooming until it's approximately two years old. Even then, only expect one to two blooms a season. Mature cacti, however, bloom profusely from July through September, when given the right growing conditions. Direct morning light and regular waterings (making sure the soil dries out in between) will give your cactus the best chance of blooming. A diluted nitrogen-rich fertilizer can also help promote blooms when given during the summer.

Common Problems With Star Cactus

The most common problem with star cactus, as with all cacti, is overwatering, which can lead to root rot. Once rot sets in, your plant may brown at the base and become mushy. This condition is nearly impossible to treat, but the best prevention is making sure the soil dries out completely between waterings. Keeping home humidity levels low can also help, and bringing potted specimens indoors during periods of rain and high humidity is advised.

FAQ
  • Is star cactus considered endangered in the wild?

    Star cactus is considered a vulnerable species due to commercial collection (despite successful propagation techniques) and habitat alternation from excessive grazing and the conversion of the plant's natural habitat into cropland.

  • Where does star cactus grow wild?

    Star cactus is native to the lower Rio Grande Valley of Texas, and the states of Nuevo León and Tamaulipas in Mexico. In Texas, this species is restricted to a 200-acre site where only 2,000 individuals exist in the wild.

  • When will my star cactus bloom?

    Cultivated garden cacti should start blooming in their second year, and then bloom successively from July through September in North American gardens.

Article Sources
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  1. Star Cactus. NatureServe Explorer.