Aunt May's garden is just a practical little garden that she used to plant vegetables. It's also the place that Uncle Ob and Summer associate the most with Aunt May though, because it is her special place and she worked with the soil and plants with such love and diligence. Although her death makes everything painful, the garden comes to be the place where they feel the closest to her; it becomes a place where they can go for healing. When Uncle Ob wants to communicate with May, he goes out into the garden to try—even when she doesn't respond, Summer feels comforted by just being in a place that meant so much to Aunt May:
So standing there in that bleak and empty garden listening to Ob make May alive again, that seemed to fix something in me that had needed fixing ever since the funeral. (5.10)
And in the end, the garden is where Uncle Ob chooses to put up all his whirligigs. It is as though Uncle Ob and Summer use the garden as a place to let go of their grief and pain and move forward, which is pretty fitting since gardens are a place where things grow.