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. As tested in Beta 1.9 Pre-release 5, Nether Wart planted on Soul Sand in the Overworld and provided with adequate 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, only if the light level is too low.

In Beta 1.9 Pre-release 4, Nether Wart will grow regardless of the light level. Placing Nether Wart directly adjacent to each other does not cause them to break while in the overworld but the Nether Wart will not grow.

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. It is also advisable to do any potion brewing in the Nether so that the Nether Wart will keep growing and there will hopefully be enough Nether Wart grown for when you need more Awkward Potions. It is also advisable that if you need to do anything other than Minecraft that you should go AFK (away from keyboard) without pausing the game. You can also open your inventory to do something else on your computer. This, however, may be dangerous if you play on any difficulty higher than Peaceful, so to protect yourself you should build a 1*1*2 (or larger) closed off area to go AFK in.

Note: Nether Wart grows faster near a lava block, similarly to wheat with water. (Citation needed)

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 2-4.

Trivia

 * Nether Wart will not break when stepped on, unlike Wheat crops, nor will Ghast balls destroy them.
 * It makes the same sound as stone when destroyed.
 * In the Minecraft files, Nether wart is called "Nether Stalk"
 * Nether Wart grows in The End.