The edge of a garden is the edge of intention. It is the spot where the gardener ceases garden-ish efforts and lets something else happen. The edge may be a wall, a fence, a drop-off, a cessation of all interventions save biannual mowing, the start of a forest, or just the line where the driveway starts. Whatever it is physically, the edge of a garden is a line in the mind of the garden designer across which garden visitors must move. What’s needed is a threshold.
The threshold of a garden ideally contrasts with the garden not so much visually as experiential-ly, bodily. If the garden is a cool shady spot, the threshold could be a moment of brightness. If the garden is a backyard that wants to feel spacious, the threshold exaggerates that sensation by being an enclosed space.
The image above is the threshold to a woodland garden. It is possible it was an accidental happenstance, that the forest just grew that dense right there, but I doubt it. I think a gardener several decades ago intentionally transplanted several evergreens to near the entrance. Standing in the shade those evergreens cast now, how could I not walk into that sunlight?