Tutorials/Kelp farming

This tutorial seeks to teach you how to farm kelp, which are plants that are naturally found underwater.

Mechanics
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.

Manual farming
Kelp can be manually farmed by placing kelp, waiting for it to grow and then breaking the kelp block above the bottom one to allow it to keep growing.

Semi-automatic farming
Kelp farming can be semi-automated by using a piston to break the second-to-bottom block, and water current to draw the broken kelp into a hopper.

Automatic farming
Kelp can be automatically farmed by detecting when it grows and then breaking the second-to-bottom block. This can be extended indefinitely upwards because kelp will keep growing until it can no longer do so.

Update: Kelp has a grow limit not defined by water depth but by a number. You can see this number by pressing F3.