Minecraft wiki entry for Kelp_farming, associated text: Kelp grows similarly to sugar cane. One stem block can be placed underwater, and then it will spontaneously grow upwards until the top of the kelp is one block below the water surface. When kelp is planted or the plant above is broken, it sets a random age from 0 to 24, which determines the maximum height the kelp is able to reach. When kelp grows, the fully grown plant will add 1 to the age from below. If a kelp plant has an age of 25, it will not grow. This means that a kelp plant can grow between 2 and 26 blocks tall.