Moon cactus

Sold everywhere as a novelty and almost never explained, this is two plants: a mutant Gymnocalycium with no chlorophyll at all, in scarlet, orange or yellow, grafted onto the green stem of a different cactus that does the photosynthesising for it. That is why the coloured ball cannot be grown on its own roots, why it scorches in the sun it looks as though it should want, and why these are often short lived. The graft union is the weak point: keep water off it, water the compost rather than the plant, and keep it almost dry through the winter. Treated properly one will last several years and flower, and the coloured head slowly outgrows the stock beneath it.