Minecraft wiki entry for Tree_farming, associated text: A newer method of caging in the wither has also been found that enables a tree farm to be build anywhere while at the same time using the wither to break the wood blocks. Although potentially more dangerous than encasing the Wither in bedrock (as in the above design), this cage has been found to be completely reliable as long as it is built correctly. It works by distracting each head with mobs without them being able to damage those mobs.