Minecraft wiki entry for Sugar_cane_farming, associated text: This design uses pistons to harvest the sugar cane. If the sugar cane is only two blocks tall, it can all be pushed into a water stream. However, if it grows any taller, the top blocks may fall down to the sand where the player can pick it up. This design is often used as the basis for fully automatic farms, however, it must be modified to push the top blocks as well or some of the sugar cane may be lost.