Houseplant · Cactaceae
Star Cactus
Astrophytum asterias
A smooth, spineless green dome freckled with soft white wool, low and round like a sand dollar set in the soil.
Bright indirect
Every 14–21 days
USDA 9–11
1–2 in · 3–6 cm
Overview
The star cactus is a small, low dome of green, smooth and completely spineless, pressed against the soil in the flattened shape of a sand dollar. It is a plant of a narrow band of south Texas and northern Mexico, sitting close to the rocky ground, and it grows so slowly that a body no wider than a coin may be years old. Across that green surface lie scattered flecks of pale wool, soft to the eye and laid out in the loose, starry pattern that gives the plant its name.
For all that it is a true cactus, its rounded body bears not a single spine, so the surface reads as smooth and unbroken to the touch. That very smoothness is what makes it look so like peyote, a resemblance that has pulled heavy illegal collection down onto it in the wild. In late spring the crown opens into yellow flowers with deep red throats, each bloom seated right at the top of the body.
Beneath that plain green skin sits a plant that asks for very little, though only the right care keeps it sound. It is one of the most protected cacti anywhere, endangered where it grows wild and closely held in trade, so any body you keep should trace back to a nursery. Get the gritty soil and the spare watering right, and the smooth green form holds firm on its own.
Care
The short version: set this smooth green dome in bright light and a sharp, gritty mix, water it deeply only once the soil has dried through, and let it sit almost dry across winter.
Light
Above all it wants bright light on that green skin. Bright, indirect light suits it, and it will take some direct sun, but the shift into strong light has to be slow, since a body grown in shade will scorch and mark if it is set straight into a hot window.
Water
Water is where the smooth body is most easily lost. Soak it deeply only after the gritty mix has dried out completely, which falls at roughly every 14 to 21 days through the spring and summer growing season. In winter keep it nearly dry, because the plump green flesh turns to rot fast if it sits wet in the cold.
Soil
Bed it in a fast-draining mineral mix, gritty and sandy or gravelly with little organic matter. A neutral pH of 6.5 to 7.5 suits it, and the coarse grit is what carries water past the roots before it can soften and rot them.
Temperature
The green body holds up across a wide span, from about 23°F to 95°F (-5°C to 35°C), though that cold low end only stands if the flesh is bone dry. Warmth swells its slow growth, while cold and wet together are what break the tissue down.
Humidity
Keep the air dry, around 20% to 40%. This is desert flesh, and still, damp air settles on the body and opens it to the rot and fungus it cannot hold off.
Feeding
Feed it lightly, and only while the body is in active growth through spring and summer. A dilute, low-nitrogen cactus fertilizer once a month is plenty, since heavy nitrogen swells soft, weak flesh that splits and rots.
Pruning
The rounded body needs no pruning to keep its shape. A sterile blade is drawn only to lift away a spent flower or a patch of rotted flesh, never to cut its form.
Repotting
Move it into a fresh pot every 2 to 3 years, or sooner once roots break from the drainage holes or the body spreads across the whole surface. Settle it into fresh gritty mix and hold off water for a few days, letting any torn roots seal over.
Size & spacing
This is a very small body. It stands only 1 to 2 inches (3 to 6 cm) tall and widens to about 2 to 6 inches (5 to 15 cm) across, and its slow swell keeps it snug in a small pot for years.
Troubleshooting
Nearly every problem this cactus meets is written across its own green body, and most of it comes back to water. The flesh may turn soft and dark, pull inward into wrinkles, or carry small tufts of white cotton in its wool. Read the surface below and treat what you see quickly.
Soft, dark, mushy patches on the body or near the base
Soft, darkened, mushy flesh is rot, and on a star cactus it traces almost always to too much water or a mix that stays wet. It usually shows first at the soil line, where damp presses against the green body. Stop watering at once, cut back to firm green tissue with a sterile blade, let the raw cut callus over in dry, airy shade, then set the plant in fresh gritty mineral mix.
Fix:
- Stop watering immediately
- Cut away all rotted tissue with a sterile blade until only firm green tissue remains
- Let the cut surfaces callus in dry, airy shade
- Repot into fresh fast-draining mineral mix and water only sparingly once healed
A wrinkled, shrunken, or sagging body
A body drawn into wrinkles points two ways. If the soil is bone dry and the roots stay firm, the flesh is simply thirsty, and a deep soak fills it back out. If the soil is still damp, the roots have likely rotted away, so treat it as rot, cutting back to sound green tissue and re-rooting in dry mineral mix.
Fix:
- If dehydrated, give a thorough soak and resume regular dry-down watering
- If roots are rotted, treat as rot: cut back to healthy tissue, callus, and re-root in dry mineral mix
White cottony tufts in the areoles with weak, off-colour growth
Those small cottony tufts sitting in the wool are mealybugs, and they nest in the woolly areoles where they blend into the plant's own fuzz. Press one to be sure: the pest smears, while the true white wool and flecks stay dry and intact. Set the cactus apart, wet each insect with a swab of 70 percent isopropyl alcohol, then repeat weekly and check the roots, since root mealybugs lodge in the soil below as well.
Fix:
- Isolate the plant
- Dab visible insects with a cotton swab dipped in 70 percent isopropyl alcohol
- Repeat weekly and inspect the roots, drenching with an appropriate insecticide if root mealybugs are present
Toxicity
The star cactus is safe around pets and people. Nothing in its green flesh is toxic, and for all its likeness to peyote it carries no mescaline. Its spineless skin cannot prick a curious nose, though the plant is not one to be eaten.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Safe to keep in the same room. A cat that nibbles the green body may drool a little or bring some up, but only because plant matter can turn a stomach, not from any toxin in the flesh.
Dogs None
No poisoning to fear. A dog that chews a piece of the fibrous flesh may feel a brief stomach upset, and with no spines on the smooth body, nothing pricks its mouth.
Horses Safe
Nothing in the green flesh is poisonous to horses, so the plant sits near them without risk.
Rabbits Safe
Harmless to rabbits. The body holds no toxin, though it is not a food plant and has no place in their diet.
Livestock Safe
Cattle, goats, and sheep can graze beside it without poisoning, since the flesh carries no harmful compounds.
Birds Safe
No toxic threat to birds. A bird that pecks at the flesh takes no chemical harm.
People Safe
Safe to handle and to keep near children. The flesh holds no mescaline despite the peyote likeness, but it is not grown for eating, so there is no cause to taste it.
Though spineless, handle this rare protected plant minimally and wash hands after contact with any houseplant; keep out of reach of pets or children inclined to chew it.
Varieties
A few named forms circulate, and each is told apart by the way the white wool and flecks sit across the green body. These are the ones to know by sight.
'Super Kabuto'
A selected cultivar so thickly strewn with large white woolly flecks that they can almost bury the green body under a coat of white.
var. nudum
A naturally fleckless form that shows a smooth, unmarked green body, recognized by Kew as its own variety.
'Ooibo Kabuto'
A form set apart by the unusually large, raised knobs of wool that stand up on its areoles.
How to propagate
There are two ways to raise more of this small green cactus, and neither is quick. One grows it true from seed, the other joins it by grafting onto a sturdier root.
SeedDemanding
Spring, in warmth · Several months to germinate and establish; years to reach flowering size
Seed is the main way and the one that holds on to the most genetic variety. Fresh seed goes onto a sterile gritty mix in spring and stays warm and humid until the tiny green bodies push up. Those seedlings are slow and quick to rot, so they are hardened off by degrees, and the wait is long: several months to establish and years more before the body reaches flowering size.
GraftingModerate
Spring-summer, during active growth · A few weeks for the graft union to take
Grafting sets the star cactus on a vigorous rootstock to speed its swelling growth and dodge the rot that takes so many seedlings, a favored move for prized cultivars. Worked in spring or summer while the body is in active growth, the union knits together in a few weeks.
Myths
Star cactus is peyote and holds mescaline.
Its low, spineless green body looks so like peyote that star peyote clings to it as a nickname, yet the two are different plants and this one holds no mescaline at all. That resemblance keeps drawing illegal collection onto a species already endangered.
A spineless body means it is tough and can be watered like a leafy houseplant.
The smooth, bare skin says nothing about how much water the plant can hold. Its fleshy green tissue rots quickly under frequent watering and needs the same dry-down and gritty soil as any desert cactus.
Wild star cactus is free to dig up or buy.
It is federally endangered and set on CITES Appendix I, so drawing plants from the wild or trading wild-sourced ones is against the law. Only plants grown on in a nursery are lawful to own.