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Trees in Urban Design

Trees and sidewalks: A truce

An Earth Day proposal: A simple change of construction material would end the battle between maturing roots and concrete.

Philadelphia Inquirer
Friday, April 22, 2011

By Thomas Hylton

In recent years, there has been a growing recognition of the value of trees in urban areas.  Trees not only beautify our cities and towns, they cleanse the air, absorb carbon dioxide, and lower ambient temperatures.

Here in the Delaware Valley, the Pennsylvania Horticultural Society recently launched an initiative to coordinate the planting of one million trees by 2020.

It’s easy to get people excited about planting new trees.  It’s far more difficult to ensure those trees will survive to maturity.  Trees need decades of growth to reach their prime, and their lifespan can last a century.  The larger the tree, the more environmental benefit it provides – one 50-year-old canopy tree, for example, can bestow more cooling power than 100 freshly planted saplings.  Big street trees are especially important in high density areas where there are few green spaces.

Street trees can be killed by insects or disease, or run over by errant cars, but by far the most common cause of their premature demise is lifted sidewalks.  Everyone’s an environmentalist until his sidewalk buckles.  Then, too often, the solution is removing the tree -- just as it is providing its maximum ecological benefit.

Rather than remove mature trees because they lift concrete sidewalks, perhaps it’s time to rethink the kind of sidewalks we use.

Concrete sidewalks typically consist of 4-inch-thick slabs poured as large blocks over a bed of gravel.  Over time, these slabs can become uneven and create lips that cause people to trip.  Tree roots are one reason – but far from the only reason -- that concrete sidewalks become uneven.  Tree roots can grow under concrete slabs and lift them up or crack them.  But normal freezing and thawing of the ground, and settling caused by subsurface pipes, can also throw concrete slabs out of kilter.

Sidewalks don’t have to consist of rigid concrete.  In fact, there is another surface, time-tested and readily available, that is far superior: asphalt. 

Because asphalt is poured in one continuous ribbon, there are no slabs to become uneven.  When a concrete slab is lifted, the only solution is jack-hammering it out and pouring a new slab at considerable expense.  Because asphalt is flexible, tree roots can grow right through it, at worst causing bulges rather than lips. And unlike concrete, cracked or lifted sections of asphalt can be easily cut out and replaced.

Asphalt is commonly used for streets, parking lots, and walking paths throughout the region.  Large portions of the Schuylkill River Trail, from Philadelphia to Valley Forge and beyond, are constructed of asphalt.  Walkways at colleges and universities – from Bryn Mawr to Princeton – are made of asphalt.  Scores of parks throughout the region have asphalt paths.

 So why not sidewalks?

Asphalt sidewalks are common throughout Europe.  They can be found in a wide swath of New England towns from Providence, R.I., to Freeport, Maine, which have much more severe winters than we do.   It you don’t like the color, asphalt can be pigmented or painted.  New York City has colored miles of asphalt bike paths, and portions of streets now closed off and used by pedestrians, with a pigmented epoxy that comes in all colors.

In my town of Pottstown, a non-profit called Trees Inc. raised nearly a half million dollars in the early 1980s to plant more than 1,500 street trees.  Thirty years later, the trees have beautifully transformed the appearance of the downtown and surrounding neighborhoods.  Some trees now soar over two-story houses and shade entire blocks.

But they have also lifted sidewalks.  And we’ve found the easiest, most cost-effective way to create safe walking surfaces without removing the trees is replacing concrete with asphalt.  Two years ago, Trees Inc. replaced severely lifted sidewalks around 15 street trees with asphalt as a demonstration project.  The tree wells, in which the trees grow, were expanded as much as possible, so few roots needed to be pruned.

The newly-poured asphalt was then coated with a permanent epoxy solution colored to blend in with concrete.  Today, the asphalt remains in pristine condition and is almost indistinguishable from the concrete.  Meanwhile, Pottstown Borough Council passed an ordinance allowing asphalt sidewalks to be used in areas where trees are planted.

Replacing lifted concrete with asphalt makes sense – but it makes more sense to avoid the use of concrete sidewalks in the first place. If we want environmentally friendly communities, we need to ensure the trees we plant last throughout their natural lifespan.  Trees and sidewalks can safely coexist.  We just need to be flexible in our thinking and our construction. 

Thomas Hylton, the author of “Save Our Land, Save Our Towns,” is co-founder and treasurer of Trees Inc.



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