Nether Wart

Nether wart (also known as Nether stalk) is a plant grown in the Nether. It can be found within Nether fortresses around staircases or simply on soul sand beaches in the Xbox 360 edition. Nether wart can be planted on soul sand, where it will grow. Nether wart is a critical ingredient in brewing as it is used to make the awkward potion, which is used as the base for most other potions.

Growth
Nether wart can only be planted on Soul sand. It can be planted and will grow in any dimension, but it can only be found naturally in the Nether. In the Xbox 360 edition, nether wart can be found anywhere soul sand is generated and can be grown in the Overworld.

When planted on soul sand, a Nether wart will grow through four stages, though the middle two stages have the same appearance (this can be verified by using a BUD switch to detect the stage transitions). Growth is driven by block ticks, similar to wheat. The duration of each stage is random, with no upper or lower limit, but the average duration is about 3 minutes and 30 seconds. The growth rate is not affected by light or any other environmental factors. Bone meal has no effect on the Nether Wart.

Farming
Nether warts are ready to harvest when they reach their third stage (when the warts appear dark crimson and there is no space between the blisters). Breaking a full grown Nether wart yields 2 to 4 Nether wart seeds, while an immature one yields only a single seed. Unlike farmland-based crops, jumping on the Nether warts will neither destroy the crop nor affect the block it's planted on.

When harvesting a large field of Nether warts, some players find it useful to construct a platform one block below the level of the soul sand, or to place the soul sand on top of the ground, with a path around it. When looking from this level, the tops of unfinished Nether warts will form a horizontal line at eye level, with mature warts protruding above.

Video
(Toby says that Nether Wart can only be grown in the Nether. In updates since, Nether Wart can be grown in all dimensions as long as it is planted on Soul Sand.)

Trivia

 * Nether Wart makes the same sound as stone when destroyed.
 * Nether Wart items count as "seeds" and can be used to breed chickens.
 * Nether Wart grows rather quickly, and you don't actually use much of it. Half-a-dozen blocks of soul sand will easily support a brewery and then some.
 * As of TU8, Nether Fortresses will spawn Nether Wart more often. The bedrock wall may, however, cut the proper room for it off, making it near impossible to find.