Tree Farming is the process of creating an environment in which saplings can safely form and mature into trees. Since planting trees is not possible, a more indirect approach must be used. This technique may be used to supplement a thriving wood industry. Tree farms require a large area and may not produce fast enough for impatient gamers due to the time requirements involved.
Creating a tree farm is generally straightforward, but depends on your situation and needs. Walling off an outside area may do, but if your fortress isn't in a thickly forested area then this may not suffice.
 Soil Layers
The fastest way to clear out space for a large tree farm is by digging out a subterranean soil layer while leaving the floors intact. This only works after a cavern has been breached. Since soil is easy mined and requires no irrigation system this is probably the easiest option if it is available. Note, however, that surface tree growth will be greatly reduced if you mine out the layer directly below the surface.
 Deep Underground
If you find a suitably sized cavern opening you can opt to just use that as a tree farm, possibly enlarging it. Walling it off provides safety against migrating "wildlife".
If the above options are not for you then your next option is to dig out a large underground space and flood it. This too only works after a cavern has been breached. Locate a suitable level and set your miners to work. While your miners and haulers are hard at work set up an irrigation system. Large amounts of water will need to be utilized since evaporation is a significant force when dealing with copious amounts of liquid spread over open areas. Rivers and lakes make excellent sources, but be wary of slowing frame rates especially with lakes. Once every tile has been covered then you can opt to recycle the water by dropping it down onto a similarly opened out space on a lower level. This way you may dig out larger farms without requiring more water.
 Use and production
Freshly watered or sporulated farms will require about a year of growing before trees pop up. In this time a few may show up early, but the majority will mature as a group within the span of a month of in game time. After it is clear cut trees will show up slightly more frequently and steadily as time progresses. It's important to leave it be as it works, so don't try to micromanage it!
Tree farms can double as pastures, but trampling may lead to lowered wood production in the area occupied by grazers. Separate pasture and tree farming areas can be created by making pastures out of up stairs to prevent the growth of trees and shrubs, but though all the tiles in the pasture can grow moss a stair tile will only support sparse moss growth.
Saplings will not grow on soil that is occupied by rocky boulders, items, buildings or stockpiles. Removal of these obstructions will increase the area saplings can grow on.
Saplings of underground trees will not form in artificial areas until a cavern has been breached. Underground saplings will die if marked as Outside .
A sapling must fulfill a few requirements in relation to the environment in order to mature. Saplings will not mature if there are many others of the same species in the area around it. On top of this, surface trees must meet a certain density requirement based on the biome your fortress is in. If there are too many mature trees in an area then growth will be halted. Underground trees always grow with uniform density.