A greenhouse is the newest power of the city you decided to recruit. You’re trying to recruit all the buildings in town and the greenhouse was the next on your list. It adds just a little bit of power to the town, but it's important nonetheless.
After you recruit the greenhouse, you recruit Bobo (ぼ) the clown to guard it for you. Bobo the clown has a terrifying amount of power so he’ll make an excellent guard for the greenhouse.