Minecraft wiki entry for Kelp_farming, associated text: Kelp can be manually farmed by placing kelp underwater, waiting for it to grow, and breaking it. Usually you will want to break the block above the bottom kelp plant so as to allow continued growth. If you do not need a large amount of kelp, such as several double chests filled, it is possible that harvesting naturally grown kelp in the ocean will turn out to be sufficient.