How Cactus Plants Are Propagated
Nothing more is necessary than to put one end of the slab in the ground. Indeed, the slab will probably germinate and send down roots even if it is merely laid on the ground. The cactus is a hardy plant and it takes a lot of rough usage to discourage it. Each of these slabs will grow presently into a large, robust plant.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 8 Chapter 6