living fence

plural living fences

1. agricultureA natural boundary formed by densely planting shrubs, trees, or vines in order to create a functional barrier.

   A short time ago the Orange Judd Farmer gave an illustration of a living fence, cedar trees for posts.

   1893, T.H. Hoskins, “Pruning Trees In Cold Climates”, in The Canadian Horticulturist, volume 16, page 31

   2 more examples

   We are not however to imagine there are no inclosures at all; they have mounds of earth-walls, or living fences, about their gardens.

   1764, Thomas Harmer, Observations On Various Passages of Scriptures, page 216

   But there is a kind of living fence which isn't a hedge and therefore merits discussion here. It's properly called an espalier.

   1976, Stanley Schuler, How to Build Fences, Gates, and Walls, page 163