Greenhouses are best described as an automated grass making machine. After you load the greenhouse with 10 tufts of grass, you will be able to collect 1 grass per 20 seconds. You can produce a maximum of 100 grass in a greenhouse before it needs to be emptied. There may be more options for greenhouses once crosbreeding is done at UoW. At this point, crossbreeding is not fully coded and therefore not in the game yet.