Nether Wart

Nether Wart is a plant grown in The Nether. It can be found within Nether Ruins 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.

Because they grow in low light levels, they cannot be grown without the potential for mobs to spawn in the farm, although. This is irrelevant however, since they must be grown in the Nether, where there are no hostile mobs. Automatic harvesting could be useful to prevent being attacked.

It is important to note that nether wart will only grow whilst you are in the nether, and therefore mass harvesting of it requires one to spend a long time in the nether to harvest it. Finding an activity to do in the nether - harvesting other potion ingredients, for example - would be advisable.

It is unknown if they will grow in a chunk that isn't currently rendered (needs testing).

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 Warts will not break when stepped on, unlike Wheat crops.