Minecraft wiki entry for Kelp, associated text: When a kelp plant block is broken, the age of the kelp plant block underneath is randomized to a value from 0 to 24 and the kelp continues growing until it reaches age 25. It is possible to use this mechanic to cultivate a kelp plant to increase its growth height beyond its natural maximum height of 26 blocks. This can be done by breaking the top-most block of the kelp plant each time it reaches age 25. A kelp plant cultivated by a player in this way repeatedly grows until it reaches the water surface.