Minecraft wiki entry for Cactus_farming, associated text: The schematic shows an efficient way of building a farm like this. The redstone and pistons should be built 1 block beneath the sand. Cacti should be placed on every sand block. A block will need to be placed on the pistons or they will not push up high enough to break the cacti. If you are lacking in slime, gravel or another falling block can be used instead of sticky pistons.