Automatic Kelp Farm — 3D Build Guide
The kelp grows, an observer sees it, a piston cuts it, the stream carries it to a hopper.
Four kelp columns in a glass box with a water stream on top, built block by block in 3D. The layout was built and run on a vanilla Minecraft 26.3 server before it got here: nothing despawns, about 30 kelp an hour.
How to build the kelp farm, step by step
- Floor — Lay a row of 13 cobblestone for the column floor, a row beside it under the glass wall and a row on the other side for the pistons to stand on.
- Glass box — Build glass five blocks high along the far side and at both ends, and two glass rows on top of the piston side. The water stays inside, the stream runs on top, and drops cannot hop over the edge.
- Pistons — Kelp stands in every other cell: cells 2, 4, 10 and 12 counting from the left. In front of each one, at the second height, place a piston facing the column; cobblestone in all the other cells.
- Observers — Put an observer on each piston, looking at the third height of its kelp cell; cobblestone in the other cells.
- Hopper and chest — In cell 7 at the third height place a hopper pointing out of the box, a second hopper in the glass wall and a chest outside. Cells 6, 7 and 8 below it are solid cobblestone.
- Fill the columns — Fill the two lower layers of every cell except the solid middle with still water.
- Temporary floor — Close the top of the columns with any blocks for now.
- The stream — Pour water at both ends of the top layer. It runs over the temporary floor to the hopper in the middle.
- Swap the floor for water — Break the temporary blocks one at a time and pour a bucket into each cell. The stream above keeps running: its level comes from its neighbours.
- Plant the kelp — Plant kelp at the bottom of the four kelp cells. The still water on both sides refills a cut cell, so drops float up instead of being pushed down.
- Redstone line — Behind the pistons lay a row of cobblestone and a line of redstone dust on it, right behind the observers.
- The farm at work — Cut kelp floats up into the stream and rides it to the hopper. About 7.4 kelp an hour per plant, about 30 an hour from this row of 4.
Kelp farm — questions
How fast does kelp grow in Minecraft?
On every random tick the top of a kelp plant has a 14% chance to grow one block, so a block appears about every 8 minutes and one plant gives about 7.4 kelp an hour. This farm has 4 plants and makes about 30 kelp an hour.
Why does the kelp stand in every other cell?
When kelp is cut, its block turns into falling water for a moment and pushes the drop down. With still water on both sides the cell becomes still water again at once, and the cut kelp floats up into the stream. When kelp stood side by side, drops hung in the columns until they despawned.
Why do I need a temporary floor?
Flowing water does not spread over still water. Without a floor the stream on top of the columns stops after one block. Pour the stream over temporary blocks first, then swap them for water one by one: the stream keeps its level from its neighbours.
Why is the hopper surrounded by solid blocks?
The bowl of a hopper is wider than its body. Kelp floating up beside or under it gets stuck under the rim and despawns. So the water may touch the hopper only from above: it stands on solid blocks with solid columns at both sides.
What is kelp used for?
Dried kelp is food, and nine dried kelp make a dried kelp block, a furnace fuel that smelts 20 items. Kelp is smelted into dried kelp in a furnace or a smoker.