Cactus blocks cannot be placed directly next to any other block. If a cactus plant should grow such that the new segment is directly next to a block, the new segment will automatically pop off. By arranging cactus to grow next to another block and surrounding it with running [[Water]] which leads to a [[Hopper]], one can make a fully-automated cactus farm, even in vanilla Minecraft. Cactus need neither light nor water to grow.
Both mobs and items which come in contact with a cactus sustain damage. For mobs, this does 1 point (half a heart) of damage. For items, it will destroy them instantly.