Gardens are man made, and are as natural a city or strip mining or treefort. It is another use of man "conquering" nature for his own means. Any reffrent to the natural in gardens is purely through man's design.

"The growing taste for ruins, mountains, torrents, for situations provoking fear and meloncholy, and for all artwork about these things suggests that life had become so placidly pleasant for England's privilaged that they could bring back as entertainment the terrors people had once strived to banish." pg 91, Wanderlust

My first memories of gardens probablly aren't of gardens proper but of the many trees which grew around my house growing up. There was a very large one, which took up a whole lot across the street from my house where I would hang out with my neighborhood friends. While not a garden we still left our mark on the tree, swinging from ropes and climbing with the help of a few boards here and there. Regardless of how little we actually used these interferences with the natural environment we prefered to climb the branches and explore the caverns which the long roots made. In anycase I find it interesting that no matter what man's (or in this case, children's) interaction he feels inclined to leave his mark.