The Forest-for-the-Trees Series:

Preventing Construction Damage to Trees.

by Paul Hetzler, ISA Certified Arborist.

From a tree’s perspective, home construction is the source of all evil. I have to admit beavers, chainsaws and forest fires aren’t exactly kind to trees. But no matter what sorts of worrisome issues a tree in your yard may develop, whether early fall leaf colour, tip dieback, slow growth, a sparse crown, or even many diseases and insect infestations, the problem in most cases can be traced back to construction or improvement projects that occurred years ago.

Root damage is especially pernicious because it takes a number of years for the harm to manifest. Three, five, or even ten years out from a construction job, few people think about root damage when their tree looks sick, because the causative event seems like ancient history. Also, root damage is essentially irreversible. By the time symptoms develop, it’s too late to save a tree.

Part of the reason root damage is widespread stems from a flawed understanding of tree biology. It is safe to say that most of us assume tree roots are deep, and that they extend as far as the canopy projection, or drip line, at most. Both those common assumptions are far from the truth.

Although oaks and nut trees have substantial taproots when young, in maturity their root systems have a profile like a pancake, the same as other tree species. The next time you notice a tree which has been blown down in a storm, look at the roots – there isn’t a monster taproot. It’s no coincidence the flat root mass one sees on a wind-thrown tree is referred to as a root plate.

About 90% of tree roots are in the top 25 centimetres of soil, and 98% are in the top 46 cm. A tree’s roots extend, barring an obstacle like a road or building, two to three times its branch length. This is a tree’s root zone: a broad, shallow, vulnerable network of roots.

Sadly, a false perception of tree biology has dreadful health implications. For trees, anyway – who knows what it may portend for our well-being. If we believe tree roots are deep and like it that way, we won’t think twice about adding fill, paving, or otherwise covering the root zone.

To survive, roots need oxygen, which can be in short supply. One would think all parts of a tree would be well-oxygenated, given that leaves make 02 through photosynthesis. But all living cells in a woody plant, those in the branches, trunk and roots, use oxygen and give off carbon dioxide. Trouble is, trees can’t move 02 from their leaves to the cells that need it – roots depend on soil pores to allow oxygen to seep in from the surface.

Soil compaction from vehicles or equipment operating within the root zone can mash those pores shut tight, especially if soil moisture is high, like it is in springtime. In wet conditions, even heavy foot traffic or compulsive Morris Dancing (let’s say) can close soil pores and exclude oxygen. When that happens, roots slowly suffocate.

Obviously, construction damage can go beyond compacted soil. Excavating or trenching within a root zone severs some tree roots and usually compacts the rest. Depending on factors such as how vital the tree was pre-construction, its age and species, whether the soil is sandy or clayey, and the extent of damage, impacted trees may begin showing symptoms one to ten years post-event, with four to five as the average. Drainage is commonly altered when a site is developed for home-building, and this can also impact tree health years later. Because of such time lags, secondary, opportunistic agents often get the blame.

A strong, happy tree is able to respond to insect feeding by producing chemicals known to scientists as “Bad-Tasting Stuff” to repel them (bugs, not scientists) at the site of feeding injury. It will endure some loss due to insect feeding, but it will be able to keep the balance in its favour. In a similar way, vigorous trees make more antimicrobial compounds at the site of a wound than root-damaged trees.

It is a fair question to ask how trees in little concrete tree pits in sidewalks manage to survive. Because they got marooned there as little tykes, they adapt to limited root space. In technical parlance, they’re “Unhappy.” If a mature tree with a normal root system suddenly has its roots cut or damaged to the size of a tree pit, the term for it is “Dead.”

To preserve trees during construction, preventive action must be taken long before the first vehicle or worker arrives. Work with an International Society of Arboriculture (ISA) Certified Arborist to create a Tree Protection Plan. Cordon off root zones with a durable fence that extends at least to the drip line (branch length). Even stockpiling material under a tree can cause root damage. If driving near trees is unavoidable, maintain coarse wood chips at a depth of 20-40 cm. in the traffic lane throughout the life of the project.

When excavation within the root zone is necessary, cut roots cleanly, flush with the trench wall. If possible, lay wet fabric over the root ends until it is time to backfill. If more than 40% of a tree’s root system is cut, it is better to remove the tree. Root damage of that extent will lead to future instability of the tree.

Mitigating injury post-construction is much less effective, and far more expensive, than tree protection. Compared to “fixing” root damage after the fact, putting toothpaste back in the tube is brilliantly successful. If the harm has been done, though, act fast – by the time symptoms show, it’ll be too late.

Hire a tree-care company (preferably one with a Certified Arborist on staff) to loosen soil with high-pressure water or air injection. Injecting mycorrhizae, beneficial fungi, at the same time has been shown to be valuable. At the very least, use a gas-powered drill to aerate on a 60-cm. grid with a soil auger 3-4 cm. in diameter and 40-50 cm. long, leaving the holes open.

Don’t create raised-bed gardens around trees, or otherwise add soil to the root zone. Avoid driving/ parking within it, too. So long as the soil is not wet, Morris dancers are probably acceptable, but not on a regular basis, and only if they first remove those darn bells first.

If part of the reason you like where you live is because of the trees, use more care in planning to save them than you would on the design of a garage, deck, or home addition. Remember that if your trees suffer root injury, it cannot be undone.

For further details on avoiding construction damage, see

Paul Hetzler has been an ISA-Certified Arborist since 1996 and is a member of ISA-Ontario, the Canadian Institute of Forestry, and the Society of American Foresters.

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