The reason for my question is this: I dug out a 13 (ish) year old tree several years ago. Most all of the roots were between the soil surface and 9" deep. There were three anchor roots that grew downward. The remainder grew outward from the tree. I think, if I had cut the roots with a plow (like in the video) at a depth of more than a couple of inches, a great majority of the trees roots would have been severed.
I am looking at using the plow to disrupt the growth of prairie grass/weeds. Most of them send runners then a tap root where a new weed grows (think of strawberry runners). Some of the weeds have tap roots that are very long, so I was thinking of plowing to at least 12" or more if possible. My original plan was to cut along one side of the tree at the beginning of winter, wait 4-6 weeks for the tree to reestablish roots, then cut along the other side. In the video they go completely around the tree.
~james