Nether Wart

Nether Wart is a plant grown in The Nether. It can be found within Nether fortresses on Soul Sand. Nether Wart in inventory can be placed onto Soul Sand by right-clicking (as if it were a seed), where it will grow.

Growth & Growing Conditions
When left alone, Nether Wart planted in Soul Sand will grow through three distinct stages. Nether Wart does not react to bonemeal. In the Beta 1.9 Pre-release, though the player can place Nether Wart on Soul Sand in the Overworld, it will not grow. As tested in Beta 1.9 Pre-release 3, Nether Wart planted on Soul Sand in the Overworld and provided with inadequate light (light level lower than 8, the Nether's natural ambient light level) will not grow. Nether Wart will automatically break when two are placed directly adjacent to each other in the Overworld.

It is important to note that Nether Wart will only grow whilst you are in the Nether, therefore mass farming requires you to stay in the Nether while it grows. Finding an activity to do in the Nether - harvesting other potion ingredients, such as Blaze Rod or Ghast Tear, for example - would be advisable.

Harvesting
There are three visual stages of development. During the first two stages, it will yield one Nether Wart. On the final, largest stage, breaking Nether Wart will produce 2-5.

Trivia

 * Nether Wart will not break when stepped on, unlike Wheat crops, nor will Ghast balls destroy them.
 * In Minecraft Pre-release 4, nether wart appear to break if there is not enough light. effectively, destroying nether patches in strongholds when unattended or when not visited. This makes the initial task of finding nether wart infinitely harder than before.