Question
We need to install a french drain, but it will require cutting along side our 50 year old pin oak tree. The 2' deep trench will need to be about 4' from the trunk of the tree in a straight line to the curb. What is the chance of it damaging or killing the tree?

Answer
There is a very good chance of at least some damage to the foliage on the side that the trench will be cut. Basically the prevailing theory (backed by data) is that you can cut a tree root if the cut is located a minimum of three times the diameter of the tree away. Diameter is measured at breast height (you'll see this referred to simply as DBH) or about four and a half feet above ground. So as an example if your oak had a DBH of 20 inches you could dig a trench no closer than 60 inches away from the base of the tree (3 x 20 = 60). Obviously the further away you can trench the better. Any closer and you not only start compromising a trees ability to live, you also compromise it's structural stability.

After trenching use a handsaw or a saw and cut the root off clean. Leaving it rough and jagged is just an invitation to disease and decay.


Root damage will manifest itself in the crown of the tree, so if you cut the roots you will eventually see dieback in the top. Oaks, depending on the species, are definitely more resilient than many other types of trees. They are able to "compartmentalize" decay, which basically means if bacteria gets into the tree and creates decay, the tree will keep it in one area rather than allowing it to spread throughout. Root damage, no matter what, is always bad.