Construction Damage: The Root of the Problem

By Paul Hetzler, ISA Certified Arborist

If April showers bring May flowers, then May flowers bring backhoes. Sure, it doesn’t rhyme, but as posies push up, construction crews and equipment also emerge, so maybe it’s true.

Those considering an outdoor project this season should be aware that for landscape trees, soil compaction or/ and disturbance is the root of all evil. I suppose chainsaws and forest fires aren’t exactly kind to trees, but when you spot a sickly tree in a park, yard, or on the roadside, root damage is the ultimate cause in nearly all cases.

It takes minutes to inflict lethal damage to a tree by adding soil, driving, or excavating within its root zone. But several years can pass before the tree gets the memo that it’s dead, as fatal root damage shows up over time.

About 90% of tree roots live in the top 10 inches of soil, and 98% are in the top 18 inches. Unless they encounter obstacles like roads or foundations, tree roots extend at least twice the branch length. This is a tree’s root zone: broad, shallow, and vulnerable.

To survive, roots get oxygen through soil pores. Compaction from vehicles or equipment within the root zone permanently (in practical terms) smashes pores shut, excluding oxygen. The wetter the soil is, the more severe the damage will be. In springtime, a single pass with a vehicle can compact the soil for many seasons to come.

Adding soil or fill in the root area has the same effect. In most of these cases, roots slowly suffocate, resulting in tree decline and eventual death. Excavation cuts some roots, and the machine likely will compact the rest. Such damage may kill a tree in a few years, but more commonly the symptoms show 3-5 years later. Due to this lag, opportunistic agents are often blamed.

Urban trees in cement tree pits survive (barely) because they’re marooned there as youngsters and adapt to limited root space. In technical parlance they’re “unhappy.” If a mature tree suddenly has its roots cut or damaged to the size of a tree pit, the term for it is “dead.”

To preserve trees, take action before the first construction vehicle or worker arrives; even stockpiling material near trees is harmful. A sturdy fence out beyond the drip line is highly recommended. If driving near trees is unavoidable, maintain 8-10 inches of wood chips in the traffic lane(s) throughout the project. Ideally, work with a Certified Arborist to write a tree-preservation plan that will be included in the contract.

Instead of trenching, ask if directional drilling is possible. If not, at least cut roots cleanly, flush with the trench wall. Lay wet fabric over exposed root ends until backfill time. If over 40% of a tree’s root system is cut, remove the tree; damage of that magnitude will lead to instability.

Mitigating damage after the fact is less effective, but if that’s the situation, act fast. By the time symptoms show, it will be too late. Soil augers and high-pressure water or air injection can loosen soils to let roots breathe again.

Paul Hetzler is an arborist and former Cornell Extension Educator with roots in northern NY.