A landscape maintenance professional once called me to discuss what effects root cutting might have on mature landscape trees. This is a very important topic, because along with other important functions, roots serve to hold a tree firmly in place. Anything that changes the structural support provided by the root system may eventually decrease the stability of the tree. Root decay and cutting large roots near the base of a tree are two things that decrease a root system's ability to support the tree. Other things that can weaken or kill roots include drought, flooding or overwatering, fill, soil compaction, gas leaks or paving over roots.

Most mature trees will not show any immediate effects of root cutting, such as wilted leaves or branch dieback, even when a fairly large number of roots are cut. Indeed, root cutting is a routine practice where landscapes are being renovated or sidewalks are being repaired. However, it's the long term effects of root cutting that needs consideration.

Roots can decay when they are attacked by microorganisms, usually fungi, that live in the soil.  The microorganisms often infect a root through a wound, but some are able to penetrate a root directly, especially if the tree has been weakened by drought or overwatering. Some root rot fungi can kill a tree before it falls, others cause living and healthy looking trees to fall. You can sometimes see trees blown over with the remains of their decayed root systems still attached.

It is often very difficult to detect rotting roots, since the problem takes place out of sight below ground. Trees infected with root rot fungi sometimes have visible fruiting structures of the fungus, called conks or mushrooms, on the trunk near the ground. If you see such signs on a large tree, consider having the tree inspected by a qualified arborist.   

If a fallen tree has a very small root system with several decayed roots, it is likely that root rot caused the tree failure.  However, sometimes a tree may fail because it simply has too shallow a root system to support its weight. A tree growing in a shallow soil, in a soil  with a high water table, or in a lawn area where the irrigation water is applied frequently and lightly may have a shallow, weak root system. A tree previously growing in a grove, where the neighboring trees have been cut down, also may have an abnormally shallow root system. The problem is very deceptive because there are usually no above ground symptoms to indicate a shallow root system.

Construction activities or trenching are especially damaging to the roots of nearby trees. Trenching and earth moving equipment used around trees often sever a large portion of the existing tree roots. Without the support of the entire root system, the tree is structurally weakened. The probability of failure increases as a greater amount of the root system is cut or damaged. 

It is usually impossible to predict the exact effect that root cutting will have on a particular tree, or when an effect will occur. A tree may fail a few months or many years following root injury, or it may never fail due to the root injury. Tree species vary in their ability to tolerate root disturbances. Also, no two root systems are exactly alike. A tree with a deep,  extensive root system will tolerate more disturbance than a neighboring tree with a poorly developed root system. In general, it is important to take every step possible to avoid cutting or damaging a tree's root system.

Despite an occasional failure, most large trees are very safe. Root systems are well designed by nature to hold trees up, regardless of the tree's size.  For the most part, they do just that.  

Ed Perry is the emeritus Environmental Horticultural Advisor for University of California Cooperative Extension (UCCE) in Stanislaus County where he worked for over 30 years.