"Trees cannot be planted in a square adjacent to buildings, walls, water, or other "hard" obstacles including other trees (they can however be planted adjacent to bushes). If planted in these areas, tree saplings will die the day after, and disappear. Occasionally, saplings will die even in open areas."
Yep, I can also confirm that you can plant trees, bushes, and bamboo directly next to the cobblestone. I have done it and they grew successfully and I don't hack.