The Drip Line
You will see the term “drip line” used a lot in the tree world. Imagine the canopy of a shade tree as an umbrella. A heavy rain hits an umbrella and runs to the edge of the umbrella and drips off the perimeter. The drip line for a tree is the area under the umbrella of leaves that stays dry. The drip line is your best guess for where feeder roots for the tree will be located, and it is an essential boundary for determining if a tree will survive when encroachment with vehicles or construction disturbs a tree.
Here’s a newly fallen tree, on its side, showing the limited depth of most feeder roots. You can see from the photo how generous you need to be with width to provide room for a healthy tree to grow. Another thing illustrated here is how a tree will send out some larger roots toward a source of water or nutrients. These provide a limited number of straps that hold the tree stable during storms.
Some experts say a tree can handle 20-30% damage to the roots within the drip line and survive, but that is extremely optimistic. Some trees are particularly sensitive to root damage and can’t stand 5% disturbance – Sweetgums and Dogwoods die quickly with any root damage. Try to avoid any damage within the drip line when digging near trees. If damage is unavoidable, then anything over 10% of the area within the drip line warrants complete tree removal, unless the tree is something really special. You should remove insignificant trees that are considerably damaged during construction to avoid having to remove them later when they becomes a hazard trees and you risk damaging surrounding plants and structures. A historic tree— however you might define that— a magnificent specimen tree, or a tree that has special significance, may be worth attempting to save when the damage reaches 20 – 30%. More than that is futile. Have a memorial service for the tree, sell the wood to make souvenirs, and replant more trees somewhere else.
You can read more about trees and tree ordinances in The Advanced Guide to Trees