Growing and harvesting bananas from your greenhouse provides gardeners with the benefit of knowing where their food has come front. You can leave the bananas on the tree until they turn slightly yellow and then carefully cut them from the stalk. Do not allow the bananas to completely ripen on the stalk, or they will attract fruit flies and other damaging insects. Another option is to cut the entire stem of the bananas off the tree and hang them in a cool dry place, protected form the sun. If your bananas are slightly soft and plump, they are ready to harvest, even if they are still slightly green in color. When harvesting directly form the tree, try to only take what you will eat immediately. The fruit still attached to the trees will not ripped as quickly as the bananas you have picked. If using a knife to harvest your fruit, try to make clean, even cuts as close to the stem as possible. Remember, a banana tree only produces fruit once, so it is important to take care care of any sprouts growing at the base of the tree, as these contain your next batch of fruit.