Kelp

"Kelp is the first plant to utilise a new kind of visual feature: Animation. Before, the only things that [were] animated were either particles or the lava/water and some other solid blocks. But we never animated a plant that used transparency. This was a new challenge for the style of Minecraft - as it has never been done before, we had no frame of reference to adapt the animation style to - so we had to find it."

- Jasper Boerstra

Kelp is an underwater plant that generates in most oceans.

Obtaining
Kelp can be mined instantly with any tool or without a tool. Breaking one part of a kelp stalk destroys all kelp blocks above it. Each block has an undetermined chance to drop a kelp item.

Natural generation
Kelp naturally generates in any ocean biomes, except in frozen, deep frozen, warm, and deep warm oceans, near and around seagrass. Each chunk has $1/18$ chance to generate a group of kelp.

Growth mechanics
Kelp can be planted on a broad variety of blocks. It grows underwater if it has either a source block of water or flowing water above it. If flowing water is above it, the kelp plant transforms the flowing water into a water source block when the plant grows into it. Neither players nor dispensers can remove the water source block a kelp grows in without breaking the kelp first.

Kelp can grow in complete darkness and does not require any light level to grow. Kelp also grows without having sky access. In Bedrock Edition, bone meal can be used to grow kelp a few blocks taller.

Kelp, when planted, is generated with a randomly chosen age value, which can be checked when pressing F3. The age value of a newly planted kelp plant varies randomly between 0 and 24. Each time the kelp grows in height by one block, the newly generated top of the kelp plant increases its age by 1. When the top block of the kelp plant reaches an age of 25, it stops growing. This means that kelp can naturally grow to a height between 2 (if the first kelp plant had an age of 24) and 26 blocks (if the first kelp plant had an age of 0).

When breaking the very top of an age 25 kelp plant, the age of the kelp plant block underneath is randomized to a value between 0 and 24 and the kelp continues to grow until it reaches age 25 again. 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 very top 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 sky limit.

Each time it receives a random tick, kelp has a 14% chance of growing.

Farming
Kelp farming is similar to farming sugar cane, although kelp must be placed underwater. Automation of harvest is easier because, as of the aquatic update, items float up in water.

Trading
Kelp can be bought from wandering traders for 3 emeralds.

Usage
Kelp can be placed underwater by hand, or anywhere by the use of commands such as. Placing it by hand gives it a random age value between 0 and 24. Kelp can only be placed in water source blocks or downward flowing water, not horizontally flowing water.

Besides for decoration, kelp can be used to turn flowing water into water source blocks, which can be useful for faster bubble column elevator creation. Any building block can be placed on top of a kelp plant, which can be useful for building underwater.

Composting
Placing kelp into a composter has a 30% chance of raising the compost level by 1.

Block states
Top only

Trivia

 * In real life, kelp isn't considered a plant, but rather an algae.
 * Minecraft's Kelp appears to be based off of the real-life Giant Kelp, given its size.