Bishop's Cap Cactus

How to care for Bishop's Cap Cactus

Light

Astrophytum myriostigma thrives in as much direct sunlight as possible. A south- or west-facing windowsill is ideal indoors, providing at least four to six hours of direct sun daily. Insufficient light causes the plant to etiolate, stretching out of its characteristic compact, ribbed form. During summer, the plant benefits from being moved outdoors to a sheltered, sunny spot, with gradual acclimatisation to prevent scorching.

Watering

Water sparingly during the growing season (spring to early autumn), allowing the compost to dry out completely between waterings. In winter, withhold water almost entirely, watering just enough once every four to six weeks to prevent the roots from desiccating fully. Overwatering is the primary cause of death in this species; always err on the side of underwatering. Use room-temperature water and avoid wetting the body of the plant, as moisture trapped in the trichomes can encourage rot.

Soil and potting

Use a very free-draining compost to prevent waterlogging. A purpose-made cactus and succulent compost is suitable, though many growers improve drainage further by mixing in up to fifty per cent perlite, coarse horticultural grit, or pumice. The mix should be pH-neutral to slightly alkaline. Good drainage is non-negotiable for this species, as it is highly susceptible to root rot in moisture-retentive growing media.

Feeding

Feed monthly during the active growing season (spring through summer) with a dilute, low-nitrogen cactus fertiliser at half the recommended strength. Excess nitrogen promotes soft, weak growth that is more susceptible to disease and pests. Do not feed during autumn or winter when the plant is resting. A fertiliser with a higher potassium and phosphorus ratio supports healthy root development and flowering.

Humidity and temperature

Astrophytum myriostigma is well adapted to low humidity and actively dislikes damp, humid environments, making it an excellent candidate for centrally heated homes. Average room temperatures of 18-25°C suit it during the growing season. In winter it benefits from a cooler, dry rest period at around 5-10°C, which also encourages flowering the following season. It cannot tolerate frost, so outdoor plants must be brought inside before temperatures drop towards zero.

Repotting

Repot every two to three years in spring, moving to a pot only marginally larger than the current one. Clay or terracotta pots are preferred over plastic as they allow moisture to evaporate through the sides and reduce the risk of overwatering. Always allow the plant to rest for a week or two after repotting before resuming watering, to allow any damaged roots to callous over. Wear thick gloves even though this species is spineless, as the ribbed edges can still be uncomfortable to handle.

Pruning

No routine pruning is required or appropriate for this species. Remove any dead or damaged tissue with a clean, sharp blade, and allow cut surfaces to callous before they come into contact with moisture. If the plant produces offsets, these can be carefully removed and propagated, though many forms of Astrophytum myriostigma are slow to offset or grow solitary.

Is Bishop's Cap Cactus toxic?

Common problems

How to propagate

Seed

Seed is the most reliable and common method of propagation for this species. Sow fresh seed in spring onto the surface of a slightly moist, gritty cactus compost in a shallow tray, cover with a thin layer of fine grit, and maintain at 21-27°C under a propagator lid or cling film until germination occurs, typically within one to three weeks.

Offsets

Some cultivated specimens, particularly the variety 'Nudum', occasionally produce lateral offsets at the base. These can be carefully detached with a clean blade once they are at least 1-2 cm in size, left to callous for two to three days, then placed on the surface of dry cactus compost and watered lightly after a further few days.

Where Bishop's Cap Cactus comes from

Native to central Mexico, San Luis Potosi, Hidalgo, Tamaulipas, Queretaro, Nuevo Leon, Coahuila.

Story and origins

The genus name Astrophytum derives from the Greek words 'astron' (star) and 'phyton' (plant), referring to the star-like appearance of the plant when viewed from above, created by the symmetrical arrangement of ribs. The species epithet 'myriostigma' comes from the Greek 'myrios' (countless, or ten thousand) and 'stigma' (mark or spot), describing the innumerable tiny white trichome scales that dot the surface of the plant. The common name Bishop's Cap or Bishop's Mitre refers to the resemblance of the ribbed, pointed top of the cactus to the ceremonial headwear worn by Roman Catholic bishops.

Astrophytum myriostigma was first formally described by the Swiss botanist Salm-Reifferscheidt-Dyck in 1845, based on specimens collected from the high semi-arid plateaux of central Mexico. It became popular with European collectors during the nineteenth-century cactus craze, when exotic succulents were enthusiastically imported and cultivated in botanical gardens and private collections. In the twentieth century, Japanese horticulturalists took a particular interest in the genus, selectively breeding cultivars with dramatically enhanced trichome patterns and unusual rib counts, elevating the plant to the status of a collector's item. Today it remains one of the most widely cultivated and recognisable cacti in the world.

Varieties to know

Astrophytum myriostigma var. nudum

Lacks the white trichome flecks entirely, leaving the body a smooth, dark green colour. Considered particularly striking and slightly rarer in cultivation.

Astrophytum myriostigma var. quadricostatum

Has four ribs rather than the typical five, giving the plant a more squarely symmetrical, cross-like cross-section when viewed from above.

Astrophytum myriostigma var. tricostatum

An unusual three-ribbed form, even rarer than the four-ribbed variety, and highly sought after by collectors.

Astrophytum myriostigma var. columnare

A taller, more columnar form with smaller trichome spots, tending to grow more cylindrical with age rather than remaining broadly globose.

Astrophytum myriostigma 'Onzuka'

A heavily flecked Japanese cultivar with trichomes merged into irregular white patches, covering much of the green surface. Highly valued in collector circles.

Fun facts

• Astrophytum myriostigma is completely spineless, which is unusual within the cactus family and is due to the absence of areolar spines, though the areoles themselves are still present as small white tufts.
• The white trichomes covering the plant's surface are thought to reflect intense sunlight and reduce water loss in its native semi-arid habitat, functioning as a natural sunscreen.
• Japanese cactus breeders have developed numerous highly ornamental cultivars with exaggerated trichome patterns, some of which command very high prices among collectors worldwide.
• The genus name Astrophytum translates from Greek as 'star plant', a reference to the star-shaped appearance when the plant is viewed from directly above.
• A single plant in good health can live for several decades and grow to over 30 cm in height and width in cultivation, though growth is slow, typically gaining only a few centimetres per year.

Bishop's Cap Cactus FAQ