Sclerocactus polyancistrus is a plant of California and Nevada, growing in Mojave desert. Plants are not really rare as the territory where they occur is huge. But it is uncommon species and despite their size plants usually hard to find when not in bloom.
It is a very desirable plant for any collector, with beautiful bi-colored tight spines, grass-blade shaped white ones and dark-red long wire-like hooks (ancistrus = hook), and showy flowers that stay open for up to a week, but considered difficult to grow unless grafted.
The key to successful growing S. polyancistrus plant a "natural" way, e.i. on it's own roots, is a pure mineral growing substrate and, more importantly, matching the rain pattern of Mojave deserts with the watering regiment. That usually means no water from May to August, or plants will easily rot, and good watering in late winter and early spring. In Mojave desert it does rain in the winter of not too often, and low temperatures with night frost preserve moisture in the soil, which stays this way wet for several months straight. Once spring days get warmer, desert plants actively grow, consuming water that was accumulated in the soil. Later, once temperatures rise more, desert soils dry out. By the cactus bloom time, May usually, no rainfall had occurred in Mojave desert for a month. Also some occasional rain in August and September is possible in unusually wet years.
When the growth requirements are well understood and properly followed, Sclerocactus polyancistrus is actually a very robust plant, growing relatively quickly and blooming reliably. As you would expect from the plant that calls home such a harsh place.