Minecraft wiki entry for Cactus, associated text: Cacti naturally grow to a height of three blocks, adding a block of height when the top cactus block has received 16 random ticks (i.e. on average every 18 minutes, but the actual rate can vary widely). Bone meal does not work on cacti to speed their growth.[1] A cactus does not need light to grow and is non-flammable. If a cactus has space directly above it, it grows even if the newly-grown block would immediately break due to adjacent blocks.