What You See in a Sidewalk

Years ago while just beginning my landscape architecture education, I was asked a question for which I had no answer. Walking down the street with friends in Madison, Wisconsin, the business major in our group casually asked why all the tree grates one sees along the street are the same size. The trees inside each grate were the same size as well, roughly 6" caliper. The response I gave was that urban beautification initiatives and streetscape renovations were relatively new trends and most trees simply had not reached significant size, so they all fit in the same grate. The response satisfied the asker, but I knew there was a better explanation that I hadn't yet grasped through my studies.

A combination bench and tree prison along the Mississippi River in New Orleans.

It took me a decade to fully grasp the implications of such a simple, off-hand question.  The short answer is that tree grate components can be swapped so that as the trunk increases in diameter, the roots can continue to be protected.  The long, complicated answer is that urban trees tend to have short lifespans, often dying before reaching a size that delivers benefits to the surrounding environment through shade, rainwater absorption, air quality and real estate values.  Everyone can imagine a tree-lined street; leafy and dappled with shade. That imagery is used constantly in media to represent "nice" places. Do we envision a similar overhead canopy when we plant trees along our streets today?  More importantly, do we invest in the basics that trees need to reach those heights? 

Trees are allotted only so much space in a sidewalk.  The rest is dedicated to doorways, intersections, hydrants, light poles, inlets, vents, and space for us to pass through it all.

It's one thing to advocate a policy that trees are important and root space is important, but that doesn't lessen the demand for space from other elements. Try telling a civil engineer to move a storm inlet on a plan because a tree wants to be in the same spot, or an electrical engineer with a photometrics plan that the streetlights should be located around the tree plan and not vice versa.

The truth is, there's not much space left in a right-of-way. There is very little native soil left below the sidewalks of a city. See David Macaulay's book Underground for a unique view below the hardscape, and you'll see that utilities and building footings take up most of the space. Trees need a minimum soil volume to grow healthy and strong, and some municipalities have written soil volume minimums into their design standards. But I have never seen a tree requirement drive streetscape design. Utility plans and project budgets decide where in a design trees can get shoved as an afterthought.

Every design professional and child should read this book.

In infrastructure, form indicates function.  When you walk over a large grate set flush into the sidewalk, you know that it's there to help city life keep moving.  The building by which you are walking or the subway tunnel deep underground needs that space to connect with the surface.  That building is WORKING; you can feel the steam as you pass.

What does the form of a standard tree box indicate? That trees don't need much room. That trees are happy to give something (shade) for nothing (space). Urban trees face a smorgasbord of stressors- pollution from air; pollution from roads; damage or vandalism from cars, bikes and humans; neglect; soil compaction; an extreme microclimate due to the surrounding concrete and asphalt; and a systemic lack of soil volume into which roots are able anchor and expand.

Street tree canopies stretch much further than the roots are able.

Let's give tree roots the run of the sidewalk width. Why not bridge the rootspace for pedestrians to pass rather than stunt the tree into a cramped box? Show me a tree grate that radiates ten feet (or more) from the trunk, letting water and air down to the soil below.  As you pass over it, you know that tree is WORKING.  Shading, filtering, absorbing, and sequestering like a boss.

Design teams and clients can do two things to invest in tree longevity: give trees space to grow, and showcase that you're doing so. Walk the walk, AND talk the talk.

Walk the walk- Make adequate soil volume happen, regardless of whether it comes in a neat cube or if space has to be carved out so that it wraps around and between obstacles like a set of unwieldy Tetris blocks. So long as the soil is contiguous, the roots will follow the path of least resistance. Helpful products and features include root barriers, aeration strips, continuous root zones and structural soil.

Don't let the location of utilities or walkways end a tree's chances of building a stable root structure. Wrap the pipe in root barrier material and continue the soil volume beyond it.  Root aeration strips or structural soil can be installed below sidewalks; both provide voids to help roots pass below otherwise inhospitable obstacles to reach nearby areas of plentiful soil. 

Typical detail of structural soil below Ft Lauderdale sidewalks courtesy of M. Duchene.

Typical detail of structural soil below Ft Lauderdale sidewalks courtesy of M. Duchene.

Talk the talk- Make tree root zones visible at the surface. Systems like Silva Cells do an amazing job of providing adequate soil volumes below while the making the visible walking surface blend seamlessly with the rest of the walkway. If done right, pedestrians don't know they're treading over tree roots. But why hide the function? Use larger grates to show the true extent of a tree's root system, so you know it's underfoot and you know the grates are allowing roots and passersby to coexist. Engage artists to emulate the roots for the passersby to appreciate, or more practically, work with grate manufacturers to expand existing models to greater diameters.

As always, put the right tree in the right place. Some trees prove more unruly than others in an urban environment. Choosing the right tree for the space will save maintenance conflicts down the road, and increase the tree's likelihood of reaching maturity.