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This tutorial seeks explain common practices for farming kelp, a plant found naturally underwater.
When smelted and crafted into dried kelp blocks, kelp can be used to smelt items. One kelp block can smelt 20 items, so a single undried kelp ends up smelting 1.22 items. This makes kelp very useful as a renewable fuel source.
Smelting kelp also grants 0.1 experience. To put this into perspective, it would take about 3,060 smelted kelp to go from level 27 to level 30.
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.
Kelp has a 14% chance of growing each random tick. This means that there is a 50% chance that the kelp will grow in the first 4.6 ticks, a 75% chance that it will grow within 9.19 ticks, and a 25% chance it will grow later. For information on random tick timings, see block tick.
In Bedrock Edition, bone meal seems to only grow kelp by 1 block each time (It is possible this will be changed. Additionally, the maximum growth of a kelp plant may be reset to the current height of the plant, hindering any further growth. Not even bone meal can make a kelp plant grow again if the maximum height becomes reset to current height.
Based on testing, a 20x20 square of kelp (400) down results in a rough 50% gain over the course of 1 minute, or about 600 kelp. This does not mean that half of the kelp grows 1 block; the random nature of the growth creates a negative logarithmic curve of how many plants reach certain heights. (L shaped curve)
Here is the results from a 256 kelp test over a period of 1 real-life minute (1,200 ticks) All kelp were made to be less than age 20 for the trial, so age did not matter as none grew to 6 tall.
|Height Of Kelp||1||2||3||4||5|
|Number Of Times Grown||0||1||2||3||4|
|Number Of Kelp Plants||106||109||33||7||1|
|Percentage of Kelp Plants||41.41%||42.58%||12.90%||2.73%||0.38%|
|Percent that reached each stage||starting||58.59%||16.02%||3.12%||0.39%|
Kelp can be manually farmed by placing kelp underwater, waiting for it to grow, and breaking it. Usually you will want to break the block above the bottom kelp plant so as to allow continued growth. If you do not need a large amount of kelp, such as several double chests filled, it is possible that harvesting naturally grown kelp in the ocean will turn out to be sufficient.
It is important to note that kelp may not always grow to max height due to the random age value mentioned above. As such, it may be desirable to harvest kelp more frequently to allow it to constantly grow. In Java Edition, this age value can be checked by pressing F3.
When harvesting kelp, you will want to swim down close to the base of the plants and sweep your view, breaking large amounts of kelp. Once you have broken a sufficient amount, swim to the surface to collect the items. Make sure to collect the items within 5 minutes or they will despawn.
Kelp only grows underwater, which creates some difficulties when trying to harvest. The difficulties can be alleviated through the use of the right equipment, however.
- A helmet enchanted with Respiration can allow you to stay underwater for longer. Such a helmet allows for up to an entire minute of time underwater. A Turtle Shell helmet would also be helpful as it allows you to instantly recover air when breaking the surface.
- Moving underwater can also be slow. This can be helped with boots enchanted with Depth Strider. A Riptide trident may also be helpful. If mining out an area for a farm, you may also want Aqua Affinity.
- Some potions can also be helpful. A potion of Water Breathing can allow you to stay underwater indefinitely and a potion of Night Vision can allow you to see much better.
- A sword with Smite may be helpful for fending off Drowned.
To help streamline the farming process, you can also build up the infrastructure within the farm itself.
- To increase visibility, you could light up the area with Jack o'Lanterns or sea pickles.
- Constructing nearby air pockets allow the player to enter and replenish breath more easily.
- A conduit is extremely helpful as it provides all players within a 32-96-block spherical range with water breathing and better vision. In addition, it also attacks nearby hostile mobs.
For the farm itself, you will probably want to clear out a large area and cover it with kelp. Ideally, the area will be at least 25 blocks deep to allow the kelp maximum growth.
It is also possible to automate collection be placing flowing water at the very top which goes into hoppers. This can be tricky however, and the kelp must not be allowed to grow into the flowing water and break the stream.
Kelp can be automatically farmed by using pistons to break the second-to-bottom kelp block. Since kelp can vary in height from 2 blocks to 26 blocks, it is most efficient to have a chamber which can handle the maximum height.
It is possible to create several kelp farms using the design shown in the schematic. For the design to work, there needs to be a way to trigger the piston. This can be done with either a clock circuit or an observer. In addition, it could also be fired manually by the player.
In Java Edition, an observer pointing facing the bottom kelp plant will detect when it grows. This can be used to fire the piston facing the second kelp block. It is also possible to have the observer above the piston. However, it is possible that the kelp will only grow to 2 block in height and never trigger the observer. This makes it necessary to also have a clock or manual way to reset the farm.
It is possible to use a flying machine to harvest large amounts of kelp all at once. This sort of design is more practical on a large scale as the kelp plants are all planted compactly next to each other.
|ilmango Flying Kelp Farm (view on YouTube)|
Bone meal design
In Java Edition it is possible to use bone meal to accelerate plant growth. Since kelp does not break instantly, it is necessary to have a delay on how fast the farm runs. Otherwise, the kelp may grow before the rest is done breaking. This design will no longer produce more than one kelp per bone meal. 
|ilmango Bone Meal Based Kelp Farm (view on YouTube)|
Due to a bug from 1.14.0, automatic kelp farms were broken. This bug thwarted that design because each time the piston broke the plant, the plant may have become affected by the bug and ceased growing at some effectively random height. Most of the time, that height would still allow it to grow high enough to be broken again. But the shortest maximum height that can occur this way is 1, so from time to time a plant would become unable to grow up in front of the piston any more. Once that happened, its bug-imposed maximum height could not change, so the plant would never grow again without intervention.
Because the bug-imposed maximum height was randomly determined, a farm would start out growing kelp at a rapid pace, but as more and more plants become stuck at maximum height 1 and stop growing, the output of the farm declined over time until ultimately it produced no kelp at all.
This bug was fixed in version 1.16.0.