Dark Green American arborvitae can be spaced four to five feet apart in a hedge, but the trees will form a wider, taller hedge. They will grow about eight inches a year and mature to about fifteen feet in height, with individual trees growing higher. A Dark Green American arborvitae hedge is wider than an emerald green hedge, so it will take up more space with its width. Dark American arborvitae makes a better hedge for privacy and to block off an undesirable view.