Minecraft – How long does it take for wheat crops to grow in Minecraft

Cactus must be placed in sand, and not adjacent to another block. Diagonally adjacent is okay.

It will grow even in complete darkness, and does not need water.

You need a light level of 9 or higher for wheat to grow. Saplings will need a light level of about 10 specifically above them, so one torch (which provides 14 light) nearby will suffice. There is a small disagreement on the wiki for what light level trees need, but 10 is the safe number. Beyond that light does not affect the rate at which they grow.

For wheat to grow quickly, however, you need to place it on hydrated farmland blocks, which meet the following constraints (from the Minecraft wiki):

1. Water at the same level, or one block above the farmland block. Water one block below will not work.
2. Water up to four blocks away, including diagonals.
3. The blocks between the farmland block and the water make no difference.

Farmland blocks are made by tilling a dirt block with a hoe.

Other constraints are that wheat will only grow on farmland blocks and trees will only grow on dirt/grass blocks. Also, trees need at least 5 blocks of open air above them to grow at all, but leaving 14 blocks will allow almost all large trees to grow (source). Space around the sapling is not required but will result in more leaves and possibly wood, so I recommend leaving an open 5x5 area that is also 14 blocks tall and centered on the sapling in order to maximize each tree's potential wood output.

Here is a good example of an underground tree farm.