In this tutorial, Susan Belsinger tells us how to harvest herbs. Don't let your basil flower because it's an annual herb and it will set seed if you do this. When you put the plant out, you have to cut the plant so it doesn't grow out to much. When you cut, more leaves will grow and it will get to a foot tall. Cut all of your basil like this, and cut it once a month. By cutting the seeds, you will get 20 cups of leaves per plant during a season. If you don't cut your plant back, you won't get a good harvest.