Main Street, Greenville, South Carolina
There’s one detail that makes a huge difference in the public realm, and that’s the humble street tree.
Landscape architect Henry Arnold once said, “Fifty percent of urban design is, street trees.” On a tree-lined street, look up; the canopy forms the ceiling of our shared public room. Arnold wrote, “An urban street without street trees is like a building without a roof.”
Cours la Reine, Paris
Shaping space while lending beauty, order
The Ladd’s Addition neighborhood in Portland
Line street trees up, and something magic happens. A tree-lined street is a deliberate intervention, an ordering of the public space, a statement in the common human language of geometry. It never fails to make a place where more people want to be.
One tree, many benefits
However, street trees are not just about making the place more beautiful and ordering the space, but also:
Shade - They shade our walks and bike rides. Street trees can lower the urban heat island effect, by as much as three to seven degrees Fahrenheit.
Legare Street, Charleston, South Carolina
Value - They make the city more economically potent. Two identical houses in otherwise similar neighborhoods will command wildly different prices, if one is on a tree-lined street and the other isn’t.
Water - They hold stormwater and clean up pollution.
CO₂ - Critically, they sponge up carbon dioxide, a key greenhouse gas tied to global warming, and then they make oxygen—and we breathe that stuff!
Missoula, Montana
Lower Cooling Costs - In summer, trees shade the buildings and that lowers cooling costs, as much as 35%, and they cut glare, leaving a delightful pattern of dappled light on our streets.
Place, Brand - Flowering trees and autumn leaves put on spectacular annual shows, drawing us to these places.
Troy, New York. Kenneth García photo
Warmth - And in winter, in temperate climates, deciduous trees lose their leaves to let light in to warm the sidewalks and buildings.
Traffic Calming - They keep us safer, calming traffic.
Habits can be broken
For part of the twentieth century, the street tree tradition faded amid compacted city soil, bad habits and poor assumptions. Convinced— by examples with meager, incorrect planting pits—that urban trees will never thrive and they’ll die young, decision-makers began to question whether they were worth the effort and expense. Public works officials started trimming budgets with the old line, “We’ll add the trees in a later phase.” Some minimalists, seeing starkness as a virtue, pulled their architecture away from trees into wide-open displays. Some landscape architects and arborists began to oppose traditionally aligned trees with close spacing, arguing that this geometry was too formal and dissimilar from the way certain trees grow in the wilderness, and pointing out that the roots under each tree need substantial space. They were willing to forgo the benefits and visual effects of allowing the upper branches to intersect with those of the adjacent trees and having more continuous shade for pedestrians. During the same period, departments of transportation were focused on speeding up and accommodating more cars rather than slowing them down. So they started to see shade trees as a liability, insisting on flimsy “frangibles” instead of the sturdy oaks, maples and elms customary in times past.
Get the planting details correct
Generous planting areas on the avenues of Chicago, Illinois
The turnaround came as landscape architects like Arnold and others realized that, with the right planting details and species choices, urban trees can indeed thrive. Much depends on the preparation of the hole in which the tree will be planted. For example, for the Live Oaks common on streets here in Florida, one key is not only allowing for a larger area of loosened soil below ground for the primary roots seeking water, but also for a larger open area up top around the tree, where fine hair-like roots seek air and filtered sunlight. Our landscape architect Jay Hood specified an innovative planting system for Park Avenue in Winter Park that freed the tree trunks from tight wells with metal grates and allowed for walkable surfaces to float above the roots and loosened soil below. Those trees have defied all expectations and grown quite tall. Many people are surprised to learn they were planted so recently.
In the last couple decades, new techniques and products have emerged to improve the success of street trees in urban areas where space is tight and impervious surfaces predominate. So-called “structural soil” and underground suspension systems have become common. These allow the roots to spread within loosened soil without ramming into excessively compacted earth, yet they solve the problematic lifting of sidewalks.
Green Blue Urban’s tree root system, undergoing installation in Thomasville, Georgia. Rick Hall photo
On wide boulevards and leafy residential streets where trees can be planted in linear, continuous landscape strips, your crew can dig a long trench for the whole tree line rather than individual circular holes. After planting and backfilling (avoiding too much compacting), this produces a larger area of loosened soil for the roots to explore. Naturally this requires thinking ahead about the alignment of underground utilities, drainage, and other details. Bottom line: Consult a competent professional on the right way to plant your street trees.
Beautiful exceptions: Cour du Commerce Saint-André in Paris, and Calle Malasia, in Buenos Aires’ Belgrano neighborhood
Exceptions to the rule
For all the reasons we’ve reviewed, on most of our streets and in most of our communities, we’re better off with lines of street trees, especially in temperate, moderately humid climates. But there are exceptions to this rule. For example, extraordinarily arid places where water is scarce or costly call for other solutions to supplying shade and visual interest, such as narrow streets with buildings brought closer together and cantilevered architectural elements that encroach beyond the building envelope. Fewer trees can also be best for far-north climates where letting more sun in during the many shortened days outranks having shade on the rare warm day. Even in a sunny place where trees grow easily, we might still opt for a unique, skinny, tree-less street here and there in a neighborhood plan, to diversify the addresses and experiences on offer. There are also many kinds of commercial streets and passages, and not all require shade trees; some replace the canopy effect with suspended fabrics, galleries, or portals; others establish the visual interest with creative signs and artwork or palms or flowers instead of shade trees. With or without trees, we also take care to keep clear lines of sight to signage and storefronts on mercantile streets. In Street Design, we also wrote about the usefulness of what Raymond Unwin called the “lean-in” tree, planted in the adjacent garden instead of within the right-of-way.
As with every other detail in city design, context is crucial.
Standard equipment for great cities
When they’re right, they’re right, which is most of the time—so street trees are not just a decorative frill, something to be cut when the budget’s tight. They’re mission-critical equipment in making good, resilient cities and towns, and that’s why they’re number ten on my list of Town Planning Stuff Everyone Needs to Know.
For more information, check out my TEDx talk (it’s a love poem to street trees), and read what John Massengale and I wrote about “The Seven Roles of the Urban Street Tree” in Street Design: The Secret to Great Cities and Towns, or read Henry Arnold’s classic, Trees in Urban Design. --Victor
Episode 10 of Town Planning Stuff Everyone Needs to Know is all about street trees. This episode is illustrated with photos, diagrams and video from Yellow Springs, Ohio; Rochester, Buffalo, Brooklyn and Manhattan, New York; Barcelona, Spain; Washington, DC; Guilin, China; Stockholm, Sweden; Chattanooga, Tennessee; Paris, France; Greenville, SC; and Miami, South Miami, Coral Gables, Lake Wales, and Winter Park, Florida. Next episode: Street-Oriented Architecture.
Pont Street, London