Preventing Root Damage in Forests and Landscapes

[Paul J Hetzler]

The Root of the Problem

It may not look like it at now, but mud season is right around the corner. In towns and villages, spring involves returning songbirds, blooming flowers, and a birth-frenzy of construction projects fresh off their winter-long gestation.

But "construction damage" does not only apply to human landscapes. In rural areas, maple production often brings tractors into the woods during mud season, and timber harvests may continue even during the period between snow melt, and dry soil conditions.

More often than not, residents and developers are on the same page in protecting mature trees from construction damage.Sadly, a great many tree-preservation efforts fail in the end, but not due to lack of political will or good intentions.

The issue is that bright engineers, architects, landscape designers, and community activists are rarely also experts in tree biology. As an example, those good folks I just mentioned would probably say it would be a problem if a forwarder or cement truck were to clip a tree as it drove by, tearing a huge divot from the trunk.

An arborist would tell you differently. She or he would explain that the real problem is the massive, irreparable, fatal but invisible harm inflicted by that heavy vehicle on the its way past the tree. A glaring wound on its stem would be of no significance whatever, because that tree has been killed. It will take 3 to 7 years for it to “realize” it is dead, however – lethal root damage shows up over time. A tree preservation plan which focuses on guarding trunks from being hit by vehicles is worthless. Construction damage to trees is root damage; there is no other kind.

About 90% of tree roots are in the top 10" (26 cm) of soil, and 98% are in the top 18" (46 cm). A tree’s roots extend, unless there’s an obstacle like a road or building, two to three times the length of its branches. This is a tree’s root zone: a broad, shallow, vulnerable mass of roots.

It’s true that trees such as oaks and walnuts have a taproot when young, but in maturity their root systems look like a pancake, not a carrot. Most of us have seen trees which have been uprooted by a storm, but that monster taproot has yet to be spotted. It’s no coincidence that the flat root system one sees on a windthrown tree is referred to as a root plate.

To survive, roots need to get oxygen directly from soil pores. Compaction from vehicles or equipment operated within the root zone will permanently compress pores and exclude oxygen. Adding soil to the root zone to raise the grade (for instance to lay sod) has the same effect. In these cases, roots slowly suffocate, and trees will eventually show symptoms of decline. In wet soil conditions, he damage potential is much, much greater.

Excavation or trenching activities within a root zone will sever some tree roots, and compact the rest. Root damage may kill a tree outright within a few years, but more commonly there will be a prolonged decline over 5-10 years. Because of this time lag, oftentimes it is secondary, opportunistic agents which get the blame.

It is a fair question to ask how street trees in little concrete squares (tree pits) in the sidewalk survive. Because they are put there when young, they adapt to available root space. In technical parlance they are deemed “unhappy.” If a mature tree with a normal root system suddenly has its roots cut or damaged to the size of a tree pit, it would be termed “dead.”

To preserve trees, one must take preventive action BEFORE the first vehicle or worker arrives. Work with an International Society of Arboriculture (ISA) Certified Arborist to cordon off root zones at least to the drip line or branch length. Even stockpiling material under a tree can cause root damage. If driving close to trees is unavoidable, maintain wood chips at a depth of 8-16" (20-40 cm) in the traffic lane(s) throughout the life of the project.

When excavation within the root zone is necessary, consider directional drilling, which can tunnel below roots. If that is not an option, try to cut roots cleanly, flush with the trench wall. Lay wet burlap over the root ends until it is time to backfill. If over 40% of a tree’s root system is cut, it is better to remove the tree. Damage of that magnitude will lead to future instability.

Mitigating damage after the fact is not as effective, but if that is the case, act quickly. By the time symptoms show up, it will be too late. Hire a Certified Arborist to loosen soil with high-pressure water or air injection. Soil injections of beneficial microbes in solution has been proven valuable.

For more information on avoiding root damage to trees, visit the ISA’s educational pages at treesaregood.org, or reach out to your Cornell Cooperative Extension office.

Paul is an Educator for CCE-St. Lawrence, has been an ISA Certified Arborist since 1996, & is a member of the SAF, NY Arborists, & the Canadian Society of Environmental Biologists.