I've had mixed results with genus Astrophytum. I've lost some of these cacti to mysterious, chunk-devouring pests—I suspect rats, but it could be squirrels or birds. I've lost others to rot… perhaps underwatering, dead roots, and then rot growing on the dead roots.
This individual Astrophytum cv
'Onzuka' is doing really well so far. It overshadows its 3-inch pot
with a full 3.5-inch diameter. I'll repot it soon. I've heard from an
advanced grower that he cuts off the taproot at a certain age, forcing
the stem to grow smaller, less rot-prone roots. Sounds radical but I
seem to recall he claimed everyone in Japan does this.
It's hard not to like Astrophytum cv 'Onzuka'. It has strange, white-speckled skin, its body is oddly geometric, and its flower is understated but beautiful.
Speaking of Japan, that's where this justifiably popular cultivar of Astrophytum myriostigma is said to originate.