JFS Engineering
Land Development and Water Resources Engineering
Rethinking Street Trees
Categories: Design Vignettes

I think it’s time to re-think our relationship with street trees.  Although large cities (like NYC) and small towns across America are gaining a renewed interest in street trees, the typical street tree is crammed into a “tree pit” that is barely 3 foot square.  A healthy tree needs to breathe free within what is known as the “drip line”, that’s the furthest extent of the branches.  The root zone out to the drip line is a key factor in both air exchange and taking water for the tree, and in an urban context the tree is typically stressed, which is why the rough-and-ready London Plane is so popular.

The new tree pit standards in New York City require their minimum tree pit be at least 5 foot by 10 feet, to be sure this is still a compromise to pedestrians. In NYC, most sidewalks are at least 10 feet wide, so the tree pit is not taking up too much room, but when we add in the vault hatches, stoops, and sidewalk cafes, there is not much left for the pedestrian.

It’s an understatement that until very recently, civil engineers didn’t get along well with trees.  Between the ravages of suburban sprawl and the removal of “vertical obstructions” from the sides of roadways, trees are a part of nature we just haven’t gotten our arms around. Let’s flip the question over for a moment and consider what trees are good for:

  • Clean air,
  • Shade,
  • Implicit barrier or demarcation,
  • Traffic calming, and
  • Property values.

It’s a fact that street trees add to property value.  Even in the urban jungle, humans have a need to see green and be able to commune with natural, living things. Some urban roadways by their nature allow comfortable speeding by drivers, and the placement of trees can regulate speeds by offering a level of discomfort to the lead-footed. As a stately barrier, it is known that cars typically cannot fell a tree on impact, and offer a natural alternative to the bollard.  Obviously, the canopy of the tree offers shade and clean air.  The tree pit itself can be constructed as miniature stormwater impoundment, and help reduce some localized flooding.

It's not wasted space, it's an opportunity to place street furniture on a solid platform.

So what’s the big deal with tree pits?  Well, they are big.  In Anytown USA, a tree pit developed to NYC standards would dominate the streetscape, if we let it.  Let’s consider for a moment that we look at using a simple device, the tree grate, as a part of the sidewalk and not just an obstruction to be avoided.  The tree grate is not just an obstruction, it can be a platform for street furniture.

I offer that a tree pit, if detailed and constructed with care, can be a positive addition to a streetscape. Since damaged concrete is typically found adjacent to an undersized tree pit, cutting in a larger tree pit and grate could double as a sidewalk repair program.  This new real estate can be either left for pedestrians, or co-located with street furniture.  A cast iron tree grate’s sheer weight can prevent nearly anything bolted to it from being stolen, and if a thoughtful pattern is selected can be adapted to different pieces over time.

To be sure, installing a tree grate costs money, but everything costs money. A moderate quality tree grate can cost about $500 to $1000 per tree to purchase and install, where concrete sidewalk costs upwards of $10 a square foot, installed.  Given that the tree pit is 5 feet by 10 feet, the cost of a tree grate is comparable to a concrete sidewalk. For the business owners on any Main Street, consider the implications of more shade and a newly repaired sidewalk in front of your shop.

Perhaps by thinking about how we can work with a tree’s needs we can find new, more creative solutions to improving our urban streets.

 

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