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“Hey, Watch How You Use That Backhoe!”

By G. Owen Yost, ASLA






G. Owen Yost, Is a registered landscape architect In private practice In Dallas, Texas. He is also a member of the American Society of Landscape Architects and has published several articles and papers on landscape architectural subjects. This article is based partially on the findings of a seminar sponsored by Treescape/Dallas, of which he is a Director.


Injuries to trees rarely signal their presence when they actually happen. The worst damage may not show up until years later. It may be in the form of branches that don’t leaf out, or a tree that never grows beyond its planted size. Perhaps the owner will never blame construction damage. In any case, the owner is less likely to contact you about further landscape architectural projects when the job you do is less than promised.

Preventing damage to trees during construction isn’t impossible, it simply takes commitment. Commitment from the owner, the landscape architect, the contractor, the subcontractors and everyone else on the design team ??" down to whoever drives the lunchwagon. To be sure, everyone who enters the construction site is a member of the design team to some degree. For example – even a concrete truck that’s rinsed out in the wrong place can affect the most pleasant design you can conceive.

In order to preserve a healthy tree, its entire environment must be preserved intact.

Many landscape architects insist that the contractor install fences around natural areas to be preserved. This defines the “construction envelope” for all to see. This is where all construction activity will take place; nothing at all inside the fence! In defining the construction envelope, remember that a forest is a complete system, with mature trees, small trees, seedlings, undergrowth, animals, wildflowers, natural grasses and vines etc. “Cleaning up” the underbrush, as is done quite often, raises the ground temperature significantly. So, in order to preserve a healthy tree, its entire environment must be preserved intact.

In preserving the natural environment, keep in mind that damage can be done in many ways.

SOIL COMPACTION

This causes a tremendous amount of delayed damage at most sites. Compaction isn’t caused just by vehicles either. Simple foot traffic, such as to a lunchwagon or time clock, can squeeze quite a bit of air out of the soil. Storage sites for bricks, drain tiles, or other equipment are critical, too. This type of damage is the kind most easily prevented by defining the construction envelope visually.

When using temporary roads, temporary parking areas and temporary storage sites, many contractors have found that laying down a fiber may, prior to any surfacing, minimizes soil compaction.

GRADE CHANGES

Raising the grade significantly around an existing tree smothers it. The roots depend on contact with both the soil, and the air between the soil particles, to live. However, in the case of a slight grade increase, damage can often be prevented by using a porous topsoil. Just remember that increasing the grade more than four inches inside the root zone is asking for trouble. This may vary slightly, since some species of trees can take a lot of fill, while others are very unforgiving.

In order to communicate this "sense of belonging" to everyone on the site, owners have used many methods.

A tree “root zone” is not necessarily the drip line but is defined as one foot of radius (the tree trunk is the centerpoint) for every inch of tree caliper. Thus, a six inch caliper tree has a circular root zone with a radius 7’ of six feet.

Lowering the grade around a tree is even more of a risk. Consider that the vast majority of a tree’s roots are with a few feet of ground level, so lowering the grade not only destroys a disproportionate amount of roots, but exposes remaining roots to drying and compaction as well. A good rule of thumb is that a cut of four inches or more calls for a retaining wall instead. Even a slight cut calls for an organic mulch such as peat moss or wood chips, to help retain moisture and stimulate regrowth of lost roots.

CHANGES IN DRAINAGE

Look at the drainage patterns that existed naturally, and how much or how little water an existing tree got. That’s what the tree is used to. Giving it a lot more water (as is often done) may be “killing it with kindness.”

Trenching, on the other hand, is often done with little regard for tree roots or drainage patterns. Be aware that trenching causes a drop in the water table, on both sides of the trench, for a considerable distance. This could be bad news for a tree whose roots are used to the water table being at a certain depth.

ROOT BREAKAGE OR REMOVAL

Many a root has lost a battle with a utility line’s trench. Almost immediately after trenching, the tree suffers. But the damage doesn’t translate into dead branches, shrunken leaves or total death until much later. Contractors should tunnel under roots (especially those larger than two inches) instead of cutting them. Then, after trenching or tunneling, backfill material is critical.

Be aware that trenching causes a drop in the water table.

No tree prospers when asked to grow into fill dirt strewn with old soda cans, grade stakes and miscellaneous chunks of masonry.

When your design calls for infringement on a root zone, some advance root pruning may be called for. Or, if overhead clearances aren’t adequate, some judicious pruning may be wise, instead of waiting for branches to be “pruned” by passing trucks.

CHANGES IN SOIL pH

We all know that certain trees do their best in soil with a certain pH. So pay special attention to fill dirt, making sure it’s compatible with the existing soil. Take the time to test its pH if you have doubts. Fill dirt that, varies more than 1 1/2 points from the pH of the existing soil should be, rejected. Also, washing out ready-mix trucks on the site, or overfilling a gas tank, can drastically change soil pH in a small area. Unfortunately, this sort of damage won’t show up until, well after the work is done and the checks have cleared.

MECHANICAL WOUNDING

These wounds are obvious. It’s a backhoe crunching through a big tree root. Or an errant bulldozer gashing a tree trunk. Or heavy equipment traveling through, not around, undergrowth and seedlings. This type of damage is easy to prevent if operators feel that they are part of a team. In order to communicate this “sense of belonging” to everyone on the site, owners have used many methods. Among these are training films, strategic signs, weekend picnics for workers and their families, even monetary rewards. The flip side is that many owners levy a fine on a contractor should a desired plant be damaged.

If pre-planning is done thoughtfully, many problems and opportunities will show up. And the time will be there to plan for them. In a nutshell, to prevent damage to existing plants during construction, it should be crystal-clear to everyone involved what the eventual result will be.



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October 20, 2019, 8:25 pm PDT

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