How Do You Get Seagrass In Minecraft

Seagrass is easily farmed by simply applying bonemeal to an underwater surface and then afk left clicking with shears.

It can be harvested using shears. In order for this to work there must be 2 water blocks above the block the bone meal is being used on and the lower one must be non flowing water. Mining the block without shears will cause it to drop nothing. Using bone meal on a non transparent block that is underwater generates seagrass on that block and its surrounding blocks.