Written by Carol Frank Friday, 08 October 2010 00:00
“The best time to plant a tree was 20 years ago. The next best time is now.” ~ Chinese Proverb
The wild storms with microbursts and twisters that swept through Great Neck in June and September wrought their destruction in minutes. It took weeks to clear the debris and restore order and it will take even more time to properly plan for reforesting our landscape. While volunteer groups are raising money and advocating for restoration of the beauteous canopies that graced our streets and government entities are budgeting for additional plantings, it is important to avoid mistakes made in the past and to strive for tree selections that are not only aesthetically pleasing, but viable as well.
What mistakes you might ask? A few years ago, we wrote an article entitled “The Dead and the Dying.” It was about the Norfolk maples that were planted in abundance 70 to 80 years ago, lining many of our streets. They had provided shade, oxygen, wildlife habitat and a grand ambiance to our neighborhoods. But street life for a tree is a hard life. Those maples survived being salted in winter snowstorms, grazed by vehicles, and parched by droughts, but they were slowly and surely defeated by Verticillium wilt, a soil-borne fungus that attacks their circulation systems. The only possible stop-gap measure was to cut off a limb or branch whose leaves were showing the distress of the wilt. And that is why we saw them being pruned into unlikely, awkward shapes that were an insult to their former graceful beauty.
And so it is timely that we turn for expert advice as we plant trees for the next generation. Dr. Nina Bassuk, professor at the Urban Horticulture Institute at Cornell University, has authored definitive works on the topic of selecting urban trees, which are applicable to suburban landscapes as well. She co-authored Aesthetics of Street Tree Selection: Visual Uniformity vs. Species Diversity with graduate student, Carol Grohs, and Peter Trowbridge, professor of Landscape Architecture at Cornell.
This information packed article, complete with illustrations, explains the appeal of picking one type of tree, referred to in horticulture parlance as “monoculture,” and planting that species throughout neighborhoods. The authors write, “The advantages to uniformity are primarily aesthetic and have a long-standing tradition over many centuries internationally. A street, lined with rows of more or less identical trees, brings to most observers a sense of order and tranquility. Even in the most heterogeneous of neighborhoods, a uniform allee of trees can have a cohesive influence, tying together diverse elements and creating a sense of neighborhood identity.”
But they go on to caution that monoculture plantings, that please our eyes, hold many disadvantages. The article states, “A quick review of disease and pest problems in street tree populations reveals numerous cases of devastation due to over-planting or the exclusive planting of a single species throughout a community. Some of the most notable examples include the American elm (Dutch elm disease), American chestnut (chestnut blight), Honey locust (honey locust plant bug), Norway maple (giant tar spot and verticillium wilt), London plane-tree (anthracnose) and crabapple (scab, fireblight, cedar apple rust, and powdery mildew). Overplanting of some popular species can also lead to serious maintenance problems. Species with characteristics such as weak wood, a tendency to develop chlorosis, girdling roots, and messy fruits can certainly be used in street tree plantings, but are only manageable when planted in moderation. Examples include Norway maple (girdling roots) and Silver maple (weak wood).”
“Girding roots” refers to a growth pattern when trees produce roots that begin to grow around the main stem of the tree and cut off or restrict the flow of water and nutrients to the tree. Improper transplanting and restricted space may intensify this problem. Some species of trees are prone to girding as well.
They recommend, “Ideally, any one species should not make up more than 5 to 10 percent of the total tree population for a neighborhood or district.” Widening the diversity lessens the risk of entire plantings being devastated by a serious pest or disease.
Dr. Bassuk points out that to simply plant a wide range of different species of trees nilly- willy on a street may be aesthetically jarring. How does one balance the need for diversity and the desire for the beauty of uniformity?
She and her team have devised a grouping of different species of trees that share certain characteristics, which make them visually harmonious.
First, trees are grouped by size and shape. Large trees are greater than 30 feet in height at 30 years. Small trees are less than 30 feet at 30 years.
Second, there are four basic shapes. The canopies of trees may grow to be either round, oval, vase or columnar. The vase shape is narrow at the base and noticeably wider at the top.
These first two criteria are primary. The third characteristic that trees may have in common is their branching density. Some are quite dense with branches in close proximity to each other while others have branches that are spaced more widely apart.
Finally, the foliage texture of trees, although subtle, creates a like pattern. Some trees have leaves that are large or small, blunt or pointed and so forth.
In short, the horticulturists at Cornell have done the hard work and have grouped species of trees together that will be compatible visually. In addition, they have indicated the type of soil and the degree of moisture that the trees within the groupings need.
Once a selection of trees has been determined, the site has practical, invaluable information about planting tenets and innovations. For example, one of the common problems in urban planting is the ongoing competition between tree roots and sidewalks. Usually the roots win and one sees the heaving of sidewalks. Horticulturists at Cornell have devised a “structural soil” which is a mix of crushed stone, clay loam and a polymer called hydrogel that keeps the stone and soil evenly mixed. This soil allows tree roots to “penetrate under sidewalks and roads without heaving pavement.” It has been used on the Cornell campus and New York City.
There is also information about assessing a site accurately and selecting appropriate trees for the space it will inhabit. For example, when trees that will grow to be large are planted underneath power lines, we end up with trees that are severely pruned and aesthetically painful to behold.
Or in areas where there is poor drainage, one might select trees that actually can tolerate that condition. Conversely, trees near heat retaining parking lots must be able to survive those adverse conditions. Putting all these elements together is like fitting an immense jigsaw puzzle together. But it is a solvable puzzle.
In fact, the Urban Horticulture Institute website at Cornell is a treasure trove of information as municipalities and homeowners move past mourning for all those venerable old friends we lost and move toward smart planting for the future.
You may visit the website at: http://www.hort.cornell.edu/uhi/outreach/index.htm.