This room must be researched before it can be used. (Grounds Keeping)
The Forestry zone serves as an automatic way of farming trees as another form of profit for prisons. The forestry zone requires Grounds Keeping to be researched.
Gardeners will plant the trees, workmen will harvest them to produce 3 Logs which can be sold or, if your prison is equipped with a functioning Workshop, then your prisoners can cut down the logs into wood, which can sell for more money or again be crafted to Superior Beds, which can be sold for even more money.
Trees will be automatically bought as soon as the forestry is placed, and replacement trees will be bought continuously from then on. One tree is planted about every 12 squares, each costs $100, so depending on the size of the forestry you may want to hold off until you have sufficient capital.
A reasonable place to assign a Forestry is the area between the inner and the outer fence of your prison, if it has such.