Astrophytum myriostigma aka Bishop's Cap

Astrophytum myriostigma is a slow-growing cactus. In the wild it can eventually reach 70–100 cm in height and 10–25 cm in diameter, but cultivated specimens typically stay much smaller — around 23–25 cm tall. Plants commonly carry five ribs (though three to eight are possible), and the shape transitions from globe-like when young to more columnar with age. Expect to wait up to six years for a seed-grown plant to produce its first flower.

This cactus tolerates brief cold snaps down to approximately -6°C (20°F), but sustained cold or wet-cold conditions cause stem rot. It is rated for USDA zones 9b–10, and the safe minimum indoor winter temperature is 5–6°C. In the wild it grows at elevations of 1,500–2,000 m in the Chihuahuan desert, experiencing cool dry winters rather than freezing wet ones. Avoid allowing temperatures to drop while the growing medium is still moist.

During the growing season (spring through summer) water moderately, allowing the soil to dry out completely between waterings. Fertilise lightly in spring and summer to support growth. In autumn, begin reducing water as temperatures drop — wet soil combined with cool temperatures is the leading cause of rot. In winter, keep the plant fully dry and above 5°C. Wild plants flower in early spring; cultivated plants most often bloom in summer.

The flowers of Astrophytum myriostigma are noted as slightly to sweetly scented. The fragrance is mild and present while the blooms are open.

Flowers appear at the apex of the plant and are bell to funnel-shaped, 4–6 cm across, with numerous creamy yellow tepals that sometimes have an orange or red base or centre. Each bloom lasts approximately two to three days. The plant is hermaphrodite but self-sterile, so a second plant is needed to produce fruit. Pollinated flowers develop into hairy reddish fruit about 2–2.5 cm in diameter.

Three botanical varieties are recognised: var. nudum, which lacks the characteristic white woolly trichome covering and shows a greener stem; var. quadricostatum, which has four ribs instead of the typical five; and var. tricostatum, a three-ribbed form. Many additional cultivated selections and hybrids exist in collections, with varying rib counts and degrees of white flocking.

In its native Mexico, the bishop's cap grows at elevations of 1,500–2,000 m in the Chihuahuan desert (states of Coahuila, Nuevo Leon, San Luis Potosi, Tamaulipas). Outdoor cultivation is suitable in USDA zones 9b–10 where winters are dry and frosts are brief and mild. In cooler or wetter climates it is best grown under glass or as a houseplant. South or west-facing aspects maximise light exposure. The plant is not invasive.

No pruning is required or beneficial for Astrophytum myriostigma. The plant is spineless, so no protective gear is needed when handling it. Remove any dead or dried flower remnants from the apex to maintain appearance and prevent rot.

Bishop's cap is a slow grower and does not need frequent repotting. Cultivated specimens rarely exceed 25 cm in height or diameter, so repotting every two to three years or when clearly pot-bound is typically sufficient. Use a very well-draining cactus compost, ideally with added grit or limestone chippings. Handle the rootball carefully and allow a few days without watering after repotting to let any damaged roots heal.

The white woolly trichomes covering the stem of Astrophytum myriostigma are a natural and important feature — avoid getting them wet or scrubbing them, as they do not recover easily. Dust can be removed with a soft dry brush. Do not use leaf-shine products or damp cloths on the stem.

Astrophytum myriostigma is most commonly propagated from seed, as the plant does not produce offsets (it grows as a solitary specimen). Because flowers are self-sterile, cross-pollination between two plants is needed to obtain viable seed. Seedlings are slow-growing and may take up to six years to reach flowering size. Some commercially available plants are grafted onto faster-growing rootstocks to accelerate growth.

Yellowing or softening of the stem in Astrophytum myriostigma typically indicates rot, most often caused by overwatering or watering during cold weather. Allow the soil to dry completely between waterings during the growing season and keep the plant completely dry in winter. Ensure the growing medium is very free-draining. If rot is detected, cut back to healthy tissue, allow to callous, and re-root in dry cactus compost.

Browning or corky patches at the base of the stem can indicate natural aging (corking), pest damage from root mealybugs, or rot. Check the roots if base browning spreads upward. Brown, dry patches higher on the stem may be sunscorch if the plant has been moved suddenly into strong direct sun.

Astrophytum myriostigma does not droop in the conventional sense, but a soft or leaning stem is a sign of root rot or severe underwatering. Root rot from excess moisture is the more common cause; check the roots and reduce watering if rot is present.

As a cactus, Astrophytum myriostigma does not have leaves and does not experience leaf drop. Any concern about the plant's appearance is more likely related to stem health, rot, or pest activity.

Slow growth is entirely normal for Astrophytum myriostigma — it is inherently a slow-growing species. If growth appears completely stalled, ensure the plant receives sufficient light (full sun) and is fertilised lightly during the growing season. Some collectors graft plants onto more vigorous rootstocks to accelerate growth, but ungrafted plants will always be slow. Inadequate light is the most common cause of unusually poor growth indoors.

The most common pests are mealybugs (visible as white cottony masses) and root mealybugs (detectable on inspection of roots during repotting). Red spider mites can also occur in hot dry conditions. Inspect plants regularly, especially root mealybugs which are easy to miss. Treat with appropriate insecticide or by rubbing off with alcohol. The main disease risk is stem or root rot caused by overwatering, especially in cool weather.

Flowers of Astrophytum myriostigma are hermaphrodite (containing both male and female parts) but are self-sterile, meaning a single plant cannot set seed on its own. Cross-pollination between two separate plants is required to produce fruit and viable seed. Pollination is carried out by insects in the wild.

Astrophytum myriostigma is not recorded as an edible plant. It is grown exclusively as an ornamental cactus. No edible parts have been documented in available sources.

No medicinal uses for Astrophytum myriostigma are documented in available reputable sources. It is not recorded in ethnobotanical databases as a medicinal plant.

The primary use of Astrophytum myriostigma is ornamental. It is widely grown in cactus and succulent collections worldwide. Its spineless nature makes it one of the more approachable cacti to handle and display. It holds the RHS Award of Garden Merit. No other significant uses are documented.