Question
I recently did some excavating near a black walnut tree where I researched the internet about how much of a root system can be cut before unrepairable damage would be caused to the tree itself. According to various sources, 20 to 25 percent of the lateral root system can be cut before the damage to the tree is too severe.  Any thoughts?  Also, there was an 8 inch root (in diameter) that was cut about 10 feet from the base of the tree which has been dripping water for about a month.  This worries me as the cut root is on the downslope of my mountain, and any tree repair mastic I buy at my local home building supply store does not seem to stop it.  In fact, it seems that every time I apply something to help seal the wound, the directions tell me to clean the area first around the root.  The problem with this is that I clean away that white gooey residue that builds up (which appears to slow the trickle of water), and it begins to leak even faster.  Is there any product you can recommend, and being it rainy season in california, will it heal eventually on its own after all the rain stops for the year?  Please help!! I do not want this tree to die.

Answer
Do not put anything on the wound the tree will heal over the wound by itself. Putting wound dressing on slows the healing process. Leave it alone and the wound will heal and the dripping will stop.

There is usually little damage from root systems that are cut back 20-25%. The tree will compensate by not growing some of the foliage back but there will be little health problems with these small root disturbances. There is a general balance between the amount of foliage and the amount of root system so if one is cut the other balances out.