A mixture of species is best for street trees.
American botanist Charles Sprague Sargent, writing in 1890 for his weekly magazine, Garden and Forest, expressed a strong opinion about street trees:
It is plain that trees of one variety only should be planted in a long, straight row parallel with the lines of buildings in a continuous street. Much of the desired effect will be lost if the trees vary in form or size, or expression, or rapidity of growth, or in the time of putting forth their leaves or shedding them.
An avenue of American Elms, with their lofty overarching tops, is always beautiful, because the charm of each tree is renewed in the next, and the effect of the whole is constantly intensified and multiplied by repetition. An avenue of stately Tuliptrees is equally beautiful, but in an entirely different way. The same might be said of a double row of Pin Oaks, where there is space for their drooping lower branches.
But if all these trees were intermingled, and Sugar Maples, Horse Chestnuts and others still were added, the result would be incongruous and contradictory. There would be no continuous lines extending through the entire vista to help the perspective and to give unity of character and expression and consistency of purpose to the whole.
And yet in our city street-planting it is the common practice to allow each lot owner to select the tree which suits his fancy. The immediate effect is bad enough, but it grows worse as years roll on and the individual trees become more and more unlike each other as their peculiar characteristics are more strongly marked with age.
(Professor C. S. Sargent, Garden and forest, Volume 3, Issue 108, March 19 1890, p.137.)
Professor Sargent was wrong!
Sargent was no stranger to trees and tree management; he was the director of Harvard University's Arnold arboretum from 1872 until his death in 1927.
Evidence of Professor Sargent's importance in the field of botany persists to this day. When you see the word Sargentii or the abbreviation Sarg. in the name of a tree or other plant, it refers to Charles Sprague Sargent.
But despite his well-deserved and long lasting fame, he was misguided in his advice that street tree plantings should be homogeneous.
When all the trees on a street are the same species, the neighborhood is at danger of a tree pest or disease wiping out every one of them, leaving the street bare and shadeless. Professor Sargent's ideas about desirable street tree plantings were conceived before the days of Dutch Elm disease, chestnut blight, and emerald ash borers. He had never seen a disease or insect sweep down a street, killing tree after tree.
Insuring diversity in street tree plantings
Today, urban foresters agree that street tree plantings should be as diverse as possible to help insure the health of the trees. Site variables, aesthetic considerations, and maintenance requirements will always limit the list of trees that may work in any particular urban location, but within those parameters, as much diversity as possible should be maintained.
A growing number of municipalities have ordinances that specify the minimum number of tree species that should be represented within a block or a development.
Due to the tree ordinance that [consulting urban forester Robert A.] Cool helped implement, new developments in Lansing [MI] are now required to have at least five different genera of trees planted on each block. No two adjacent trees can be the same genus.
"If you plant a block in one type of tree, in five years some will grow faster and others will grow slower. Some might die. So you've lost your uniform effect, anyway," says Cool. "If another epidemic like Dutch elm disease comes along, you've lost your whole block. Mixing your plantings is like hedging your bets."
(Helen M. Stone, "Municipal Arboriculture: Street Tree Smarts")
Is your urban forest diversified?
Does your town or city have guidelines for diversity in the planting of street trees? When you drive through your neighborhood or subdivision, do you observe a variety of species in the street trees, or are they all the same? The answers to questions of this sort may indicate the future health of the urban forest where you live.