Gardens are excellent places for learning in multiple disciplines. Math, reading, science and language are all easily taught in a single lesson. And of course social science, nutrition and health are obvious topic matters in a garden where food is grown.
Here is a link to an article from the Creative Educator. It is an overview of Project-based Learning as covered in the book “Fires in the Mind – What kids tell us about motivation and mastery” by Kathleen Cushman. They have other great resources as well!