Why Street Trees?

Tree-lined boulevards from the Arc de Triomphe, Paris

Tree-lined boulevards from the Arc de Triomphe, Paris. Author’s photographs

But trees… alone form a canopy over us, and a varied frame to all other subjects, which they improve. In beauty, they not only far excel everything of inanimate nature, but their beauty is complete and perfect in itself; while that of almost every object requires their assistance.

Uvedale Price, quoted by Nan Fairbrother in The Nature of Landscape Design

Interest in urban tree-planting programs is at an all-time high today as politicians and members of the public alike jump on the dual bandwagons of support for revitalizing downtown business districts and saving our ravaged environment. Cities are developing comprehensive street tree plans and spending large sums to plant trees along city streets and thoroughfares. Citizen advocacy groups organize the planting of trees in urban neighborhoods and raise public consciousness about the importance of trees and their proper care.

But while we spend millions of dollars to plant trees along city streets, we are often condemning them to slow torture and death. Many street trees are planted in astonishingly small holes in the sidewalk or in parkway strips too narrow to support growth. Their lower branches are removed by city crews to allow passage of trucks, their tops are lopped off to avoid interference with utility lines, and their roots are cut in a futile attempt to keep them from destroying pavement. Newly planted trees often are minimally supported or tied too tightly to their stakes, and they are planted in soil that has lain barren and compacted for decades.

We accept street trees as normal fixtures of the fabric of our towns and cities. But where did the tradition of planting trees along streets begin? Are we perpetuating a custom that no longer makes sense in the increasingly inhospitable street environment? Have the reasons that prompted people to plant trees along streets changed? And would we be better off planting trees in new patterns or new ways to increase the chances of their survival and to better enhance our urban environment?

Street Trees in History

The conscious use of trees in cities has been with us only during the last three centuries — a brief period in the history of civilization. The earliest evidence of trees planted for aesthetic value dates back nearly 3,000 years to the gardens of Egyptian nobility, but few trees were found in cities except for their chance occurrence at a public water source.

Arcaded street in Padua, Italy. Author’s photographs

Arcaded street in Padua, Italy

Until the Renaissance, cities were quite small, set within the natural landscape. City and countryside were dearly defined by fortified walls and gates. Sometimes, as cities expanded, bits of nature were incorporated — a few trees, a small stream, or a pond. But cities in general were treeless. Streets were narrow, and closely spaced buildings shaded each other. Arcades, still seen in cities such as Padua, Italy, cooled the “sidewalks” and adjacent rooms. There was no space and little need for urban trees, as nature was close at hand beyond the city walls.

There is some evidence that trees were planted along ceremonial ways in ancient Greece. Along with trees such as the plane (Plateaus orientalis) found in the marketplace or agora, these were perhaps the first urban trees purposefully planted to enhance city streets and public spaces.

Occasionally trees were planted along roads or canals outside cities in Europe. These plantings were strictly utilitarian, providing shade along the way. In some places in Europe fruit trees were planted, adding sustenance for both travelers and inhabitants. Sometimes these tree-lined canals and roads extended into the cities, but they were found mostly in the countryside. Remnants are still seen in Italy in great country avenues of stone pines (Pinus pinea) or in France in lanes of plane trees or poplars.

In medieval and early renaissance times the lonely mountains and dark forests of nature were feared and walled out from the city, which was a symbol of safety and security. In England a royal edict required the removal of all trees and shrubbery to a distance of 200 feet on both sides of main roadways to discourage attacks on travelers by highwaymen.

On the whole, the deliberate planting of trees and other greenery was not a part of urban design until the seventeenth century. In Paris the military began to move their fortifications further outside the city to allow better defense against newly developed weaponry. Areas of the city vacated by the army became some of the first plazas and squares. These places, however, contained no trees until the appearance of two urban innovations designed for public recreation; the mail (mall) and the cours (promenade). Public recreation was not a new idea, but these two kinds of public space had one novelty in common; the introduction of tree planting to cities.

Before the regency of Marie de Medici, there was no way of enjoying the outdoors except on foot and in gardens. To encourage the habit of promenading en carosse in the cool hours of the evening, she ordered allées of trees to be planted along the Seine to the west of the Tuileries. Cours had existed for some time, but the idea of introducing the refreshment of nature to the promenade was new. Thus we find the beginnings of planted walkways and streets integrated into the structure of the city.

The mail was a more active place — a facility for a sport similar to croquet. The game was played with ball, mallet, and wickets on a court that was heavily planted with trees. One such mail in Paris was rented to a Florentine for the game of pamal. Under the conditions of the lease the allées were to be planted with elms having branches no lower than ten feet above the ground. This probably was the first urban tree “ordinance.”

In both the mail and the cours trees were not merely decorative but functional, providing shade and spatial definition. These first integrations of nature into the city were carefully controlled, reflecting the philosophy of man’ s dominance over nature for his own needs and pursuits. This philosophy is exemplified as well in the subsequent planting of Versailles and in the grand Parisian avenues and boulevards created in the seventeenth century under Louis XIV and in the nineteenth century under Napoleon III. The boulevards of seventeenth-century Paris were intended for pedestrians and designed as gardens, quite unlike the tree-lined vehicular boulevards created for Napoleon III. Nevertheless, both were forerunners of modern street and boulevard planting and were the first large-scale attempts to bring nature into the fabric of urban public spaces.

Russell Square, London

Russell Square, London

Nature in English Cities

In England quite a different concept of public planting was emerging. Whereas French landscape design reflected man’s control over nature, the English style leaned more toward a desire for comfort and privacy. In sharp contrast to the French, with their grand Parisian boulevards, the English were experimenting with residential districts in which short streets began and ended in green spaces with through traffic routed around them. Within these residential neighborhoods were geometric open spaces — squares, ovals, rectangles — often fenced off from the street and planted simply with lawn or sometimes with lawn and trees. Occasionally these spaces became dumping grounds for garbage and filth and led to a deterioration of the neighborhood.

These geometric spaces later took on organic forms, as in Kensington, where great crescent-shaped buildings formed irregular open spaces between them. The landscape likewise was more naturalistic, and trees were liberally planted. Such planted spaces, romantic in their expression of nature, created comfortable surroundings for residential units. Their large, spreading shade trees also offered a sense of privacy for residents of buildings that were similar in purpose to today’s apartment houses.

And so we see distinct styles of urban planning on either side of the channel. The London squares were designed to be lived in comfortably, with busy thoroughfares excluded. Planting was concentrated in the squares rather than strung out along streets. In Paris the seemingly endless boulevard dominated.

Napoleon III attempted to integrate the two approaches in bold new plans for Paris carried out by Baron von Haussmann. New boulevards and wide tree-lined streets slashed open the city, bringing light and fresh air to the crowded slums. An ingenious tree-lifting machine enabled the transplanting of thirty­-foot trees from nearby woods, transforming streets into shady thoroughfares almost overnight. It was not coincidental that these wide streets were also helpful in controlling riots and street uprisings.

Tree Planting in American Cities

In America the patterns of urban planning and planting repeated some of the traditions of tree-lined streets and squares in Europe during the eighteenth and nineteenth centuries. But the earliest American villages were modest replicas of the fortified medieval towns of Europe. Trees were essentially excluded except to provide fruit. The village common was no more than a grassy plain to pasture cattle and provide a clear view of attackers. Only later were these spaces planted, becoming parks and public gathering places.

Treeless street in Pittsburgh, Pennsylvania

Treeless street in Pittsburgh, Pennsylvania

As in Europe, street trees were later introductions into towns and cities of America, but they were planted along country lanes for several reasons. They served as a guide for horses whose drivers drifted off to sleep from weariness or over-indulgence at the pub. They reduced dust by shading roadways and causing them to dry out more slowly. And they edged cultivated fields with markers that guided farmers turning their teams at the end of plowed rows.

In the expanding cities of the eastern United States the “tree lawn” between the sidewalk and the often muddy street played an important role in protecting pedestrians from splashes by passing vehicles. Trees also were planted in these wide strips to shade passersby and protect horses tied to rails from the hot summer sun. The trees were often badly damaged by bored and hungry horses, leading to the creation of the iron tree guard. Curiously, these are still used as ornamental features around urban street trees, though their original purpose is long forgotten and today’s trees are more often damaged by girdling of trunks attempting to expand within their iron prisons.

Street tree planting in American towns and cities has been the object of emotions varying from passionate support by town boosters to disdain on the part of speculators and urban planners. In the burgeoning eastern and midwestern cities of the late nineteenth and early twentieth centuries, trees were removed for street widening almost as quickly as they were planted. Landscape architects Horace W.S. Cleveland and Frederick Law Olmsted cham­pioned the cause of planting street trees, not simply for cosmetic beautification, but to promote the health and well-being of inhabitants and to reinforce the visible order and enhance the character of the city.

Olmsted pleaded for the inclusion of trees in plans for the rapidly expanding cities: “Would trees, for seclusion and shade and beauty, be out of place…by the side of certain of our streets?… The change both of scene and of air which would be obtained by people engaged for the most part in the necessarily confined interior commercial parts of the town, on passing into a street of this character after the trees had become stately and graceful, would be worth a good deal.” These exhortations were only nominally heeded. Urbanists became more interested in buildings than in trees and open space. Their opponents, with their ingrained disdain of cities, dreamed of new utopias where city and country were merged in total harmony.

In the western United States, where there was space for utopian dreams to be realized, the same patterns of city development were generally repeated. But in southern California street tree planting did become an essential component of urban design. Perhaps it was the good soil and benign climate, or maybe it was the need to mark territory and connecting routes in the vast landscape or to lure prospective buyers to a barren plain. Whatever the reasons, street trees were planted on a scale never seen in the eastern part of the country. Whole towns were plotted and trees planted along every street before houses were built. A 1912 photograph of Beverly Hills shows tree-lined streets lacing the rolling hills with very few houses in sight.

Euclid Avenue, Ontario, California

Euclid Avenue, Ontario, California

The settlers here were tree planters. The Chaffee brothers planted miles of streets in their colony of Ontario with peppers, eucalypts, grevilleas, and palms. Euclid Avenue still links Ontario and Upland with the San Gabriel Mountains in a magnificent sweep of grevilleas and pepper trees. The Bixbys of Rancho Los Alamitos lined Anaheim Road with pepper trees to tie Long Beach to Los Angeles. Each town tried to outdo the next as a garden spot replete with tree-lined streets. Most of the trees seemed exotic and foreign to easterners lured by boosters and speculators to settle in the southland.

But in some places, such as Riverside, people began removing street trees. Newcomers insisted that they came to raise oranges, not street trees, and others claimed that they wanted sunshine, not shade. Businessmen held that trees were out of place on busy commercial streets. By 1904 so many trees had been removed that the city adopted the first municipal street tree ordinance in the West at the urging of J.H. Reed, an early fruit grower. During the next two years the chamber of commerce planted 1,350 street trees, claiming by 1905 to have the best street tree plantings of any city on the coast.

Selecting the Right Street Tree

The postwar expansion of California cities led to street tree management problems that are just now being recognized. Subdivisions were laid out by developers trying to squeeze every inch for profit. Miniature parkway strips (descendants of the tree lawn) of less than three feet wide, and sometimes as narrow as eighteen inches, were planted with large trees such as Modesto ash, Shamel ash, sweet gum, and London plane. In many downtown areas large, spreading trees were planted in tiny cutouts in concrete paving, where expansion of trunks and roots has played havoc with curbs, gutters, and sidewalks. Some trees thought to be ideal for lining streets (Modesto ash, honey locust) have since developed disease and pest problems, while others that do well in garden settings (magnolia, tulip tree) are dying slowly in the parched and polluted environment of the urban street.

We are still learning. A 1950 Sunset article entitled The Best Trees for Your Street pro­moted, innocently enough, some of those trees now known to perform poorly along our streets. But some of today’s “best” trees, similarly, are beginning to show signs of imperfection: the 1990 chill flattened the Indian laurel fig (Ficus microcarpa) in northern California; aphids and their sticky mess have soured many cities on the tulip tree (Liriodendron tulipifera); and the ash white fly clouds the future of the Raywood ash (Fraxinus oxycarpa ‘Raywood’).

And despite a growing awareness of the need to plant the right tree in the right place, we still often plant the wrong ones. There is a curious social politic about street trees. They are at once in the public domain and impinge on private property, whether in front of a residence or a store, and in that domain everyone has an opinion. Shopkeepers may object to low-branching street trees that block their stores’ signs; older people may argue that deciduous trees drop too many leaves in their yards; city maintenance crews may counter that evergreen trees drop leaves all year round; garden club members or the city council may want flowering trees to brighten the city; and landscape architects may want trees that look as though they belong in urban settings.

There are, it seems, no ideal street trees. Selection must be as objective as possible and take into consideration a multitude of factors. Tree form relative to street space, texture and size relative to scale, and seasonal effects are some of the aesthetic values to consider. Growth rate, requirements for soil and water, tolerance of heat, wind, and smog, and resistance to pests and diseases are some of the physical and environmental factors that should guide selection, Fruit, leaf, and twig drop, brittleness, and root characteristics are important maintenance concerns.

For each situation a matrix of these and other local considerations can be developed in an effort to evaluate trees for suitability for street planting. Personal preference need not be eliminated from the list of factors to consider, but it should be among the last, not the first, criteria for selection after key environmental, design, and maintenance factors have been taken into account.

One essential consideration that has only recently come to be well understood is soil volume. Too often in the past trees have been treated as posts stuck in the ground with no thought given to root space. The broad tree lawns in eastern cities of the 1850s or along Euclid Avenue in Ontario, California, probably consisted of excellent uncompacted soil. Holes didn’t need to be large, and trees grew well.

Today the average life of an urban street tree is estimated at ten years. According to Annapolis landscape architect James Urban, who has been studying why trees planted over the past quarter century or so have failed, the primary reason is inadequate soil volume or root space. Under ideal conditions the root system of a typical ten- to fifteen-inch-diameter tree would occupy a volume of soil twenty by twenty by three feet, or 1,200 cubic feet. A two-inch-diameter tree with only fifteen cubic feet of soil is generally planted in a four by four by three foot deep pit, giving the tree only forty-eight cubic feet in which to grow. Thus entombed in airless, compacted soil, the tree has little opportunity to flourish. No wonder tree roots crack pavement as they reach out at the surface for air and water.

James Urban calculates that for a tree to reach twenty-five inches in diameter, more than 1,000 cubic feet of rooting space is needed. This means that urban tree planters will have to find new ways to plant trees if we are to see any long-term success and stability in urban tree planting schemes. We can no longer plant trees in parkway strips less than three feet wide or in three-by-three-foot pavement cutouts and expect them to reach mature size. New standards for soil preparation (holes three or four times instead of twice the diameter of the root ball), channels of good soil for roots to follow under sidewalks, gravel beds immediately beneath concrete sidewalks for air, and grouping trees in shared soil volumes are a few of the innovations now recommended to help sustain the health and prolong the life of urban street trees.