Minecraft wiki entry for Kelp_farming, associated text: In Java Edition, an observer pointing facing the bottom kelp plant will detect when it grows. This can be used to fire the piston facing the second kelp block. It is also possible to have the observer above the piston. However, it is possible that the kelp will only grow to 2 block in height and never trigger the observer. This makes it necessary to also have a clock or manual way to reset the farm.