Greenhouse is that ruined building next to your farmhouse. At the beginning of the game it is not possible to use this greenhouse because it is in need of repair. There are two ways to repair the Greenhouse. You can complete all Pantry bundles in the Community Centre or you can pay 35 000 g via Joja Community Development form if you decided to get Joja membership in the first place.
Greenhouse is a really beneficial upgrade to your farm since you can grow most crops, plants and fruit trees regardless the season. Everything will thrive and bear fruit. Crows and lightnings are no threat to your crops inside the greenhouse.
Basically, if you arrange your sprinklers in a smart way, you will only have to go to your greenhouse to sow and harvest. Otherwise you don’t need to worry about what is happening there.
Don’t forget that you can also plant fruit-bearing trees inside the Greenhouse and fruit-trees grow just fine outside the wooden square, just make sure you plant them no closer than two squares to one another.
