Kelp Farm

Kelp grows in tall single-block columns underwater, basically sugarcane's aquatic cousin, and uses the exact same observer-piston harvest. An observer watches the growth point one block above the base, and when kelp grows into it, a piston knocks the new growth loose while the base survives to keep going. The only real difference is the whole thing lives inside a water column instead of beside one.

Since kelp needs full submersion to grow, each column sits in its own water-filled shaft, usually glass-walled so several columns can sit next to each other without their water interfering, and so you can tell at a glance which ones are ready.

This is genuinely one of the fastest-growing renewable plants in the game, outpacing both sugarcane and bamboo per column, which is why even a modest kelp farm produces a steady, high volume without needing many parallel columns the way a fuel-focused bamboo farm does.

The real reason to pick this over bamboo for fuel comes down to smelting math: kelp has to dry into a block first (9 kelp per block), but a dried kelp block then smelts a comparable number of items to a full stack of coal. A busy kelp-into-block pipeline is one of the more space-efficient fuel sources around, especially parked right next to a smoker or blast furnace.

Raw kelp is also edible straight off the farm without cooking, restoring a small amount of hunger, so it doubles as a reasonable emergency food source even before any of it gets turned into fuel.