Printer Friendly

Giving trees room to grow.

n Part 1 of this two-part series, we explored the major features of the urban environment and how they contribute to the underlying ecological landscape. The spotlight was placed on the complex web of natural cycles and how soil, water, flora, and fauna interact to create the natural environment. Also examined were the design and layout of a community, and how community development can displace or incorporate the elements of the natural landscape. Skillful planning followed by careful construction can make use of the existing landscape and fit new structures nicely into the natural lay of the land.

The trend in established communities has been to engineer independently of the existing landscape. Hills and streams have been viewed as problems to move or reroute rather than as parts of the overall pattern. Where the existing vegetation has been removed, the challenge is one of "re-greening"-regenerating the trees and shrubs and bringing life back to the landscape. Re-greening is a difficult process because the new plantings do not benefit from rich soils and gentle topography. Often they have to survive low soil moisture at dry times of the year and flooding caused by engineering techniques that short-circuit natural cycles.

Once a community is built, the rebuilding of its natural systems has to rely on the limited collection of tree-planting spaces that remain in yards, parks, and other open spaces. Regreening is a difficult challenge, technically and financially. This article is directed at two parts of that challenge. First it focuses on the changes needed in the way we plant trees in the regreening process; then it addresses the notion of making space for trees before a community is established-while streets are being planned, constructed, or reconstructed.

Planting a tree is an act simple enough that millions of elementary school kids across the country are turned loose to do it on Arbor Day each year. These trees are small, usually seedlings, but the event is often enormously effective as an educational event. Establishing trees in the community as a permanent part of the landscape is a much more complex job.

Hundreds of thousands of trees are planted in urban and suburban America each year, but only a small percentage of them win be alive at age 40. The closer a tree is planted to the center of the city, the shorter its lifespan. Those planted in sidewalk pits in center city will be lucky to live 10 years, even if planted by skilled experts.

What are we doing wrong? Why is it so hard to get trees to live in the city? How can a planting event that starts with such enthusiasm, dedication, and hope come to such an untimely end? Observations by many experts identify restricted tree space and outmoded planting methods as the main causes of tree decline. Other practices may complicate survival, including overzealous lawn grooming in

which mowers are permitted to level everything in sight or bang into trees, and the crude "trimming" done to street trees by metro buses and trucks. However, the focus of this article will be planting space and methods.

Just what do we know about tree planting? Planting seedlings on a farm is an everyday occurrence each spring. It's a relatively simple job: a spade or tractor opens a hole in the soil, the roots are positioned properly and covered with dirt, the seedling is watered as needed, and the planting is complete. But seedlings are usually too small to survive the active urban and community environment, so larger trees are recommended for planting there. Traditional planting specifications-well documented in landscape journals, books, and nursery catalogues-read something like this:

Dig a hole about six inches wider than the root ball and a few inches deeper; place some loose dirt and peat moss at the bottom of the tree; backfill the hole with a mix of peat moss and dirt. Build a berm of dirt around the edge of the planting hole to hold water; stake the tree, and mulch.

Although these planting specs have evolved only over the last decade or so, today almost none of these recommendations are correct. This time-tested formula was originally designed as a standard for planting on large estate grounds where soil was good and the planting conditions were favorable. Use this same method to plant in today's rock-hard urban soils, and you create a nightmare for the tree-it is doomed to a slow death by drowning or suffocation.

Nowhere are these problems more pronounced than in the city center, where trees are planted in standard pits. The pits usually measure 4 x 4 feet on the surface and three feet deep. Such a pit (or tomb, if you're up on planting humor) will hold 48 square feet of soil, air, and water. The poorer the surrounding soil, the more tomblike the pit becomes. The impervious walls hold moisture in and defy penetration by tree roots. The situation is much like the large houseplant kept in a too-small pot-the roots are forced to encircle and strangle the stem. Although funds have not been available to do the kind of research needed to identify all the causes of tree decline in cities, most experts agree that a lack of space is the overriding issue. Solving this problem has become more pressing with the heightened concern about global warming and the desire to cool urban "heat islands." To address these concerns, our goal should be to grow more and larger trees in the city. That means changing the designs for city streets as well as the specifications for planting and the size of spaces provided. If we want trees to reach a size of 15 inches in diameter, we will have to create a lot more space for roots to grow.

Of all the survival problems that trees face in cities, the ones we don't see are perhaps the most limiting. Research on root systems has begun to shed light on some of these underground problems. As already discussed, there is a fundamental problem with the planting pits we build. Tree-root systems are naturally shallow and widespreading, not deep and tightly packed around the root ball used to transplant trees. As roots develop, almost none grow below three feet. They normally spread out horizontally two to four times the height of the tree. The roots of a 10- to 15inch-diameter tree would easily take up a block of soil 20 x 20 x 3 feet or 1,200 cubic feet.

When a two-inch-diameter tree is transplanted, it carries only 15 cubic feet of soil with it. If all goes well, the transplant will fill the average 4 x 4foot pit (48 cubic feet) in about five years, and then enter a state of decline that will be unstoppable. Add a little road salt, chemical debris, mechanical abuse, and/or air pollution, and you see why the average tree in center city lives no longer than five to 12 years.

So where do we go from here? What new planting specs should be used? First, divide urban plantings into three categories: pit, street lawn, and residential. The first rule of thumb is to assign the largest potential root space possible for each tree. With suburban plantings, the ideal situation is to prepare the site with a rototiller rather than a shovel. This opens up a large rooting area near the surface (most of the roots will be in the top foot of soil) and eliminates the need to dig deep into hard, airless soils where tree roots won't grow. f the soil has a lot of clay or gravel or is easily compacted, incorporate organic matter with the roto-tiller before planting. It is important that the soil be consistent from the center of the planting hole to the outermost location you want the roots to reach. If the soil is good, you need not add anything, but rototilling is still recommended. The tree should be set in the hole uniformly, resting on a solid piece of ground so it doesn't sink after the first rain, with the top of the root ball at ground line and the trunk at a right angle to the earth. Only then is the soil backfilled.

In the average street lawn, space will be more limited than in suburban and rural plantings and may affect tree growth and survival. The best option on an existing site is to till up as much of the tree lawn as possible, and incorporate good soil or organic matter if needed (and it almost always will be). If the sidewalk has not yet been put in place, create a space for growth underneath so that roots can reach the reservoir of soil in the lawn area without destroying the sidewalk. By digging a four-foot-wide by three-foot-deep trench just opposite the tree and backfilling with loosely compacted soil, tree roots will be encouraged to grow through the passage deep enough so that the walk won't be destroyed. Another option is to supply oxygen to the roots through the installation of a gravel bed before the sidewalk is built.

Perhaps the biggest challenge is planting in a sidewalk pit where space is at a premium. Without question, the techniques in general use today are not good enough and need to be improved dramatically. The drawing on page 62 is our best attempt at creating a better tree-planting specification. Some changes must come in the way we engineer streets and sidewalks. We will have to find innovative ways to maintain air space and organic matter in the planting pits, as well as ways to increase the total volume of growing space within. With these cautions in mind, we propose this planting model as an improvement over existing standards, and suggest that future streetscape designs be adapted to consider these ideas as soon as possible.

One imaginative design grew out of a notion John F. Kennedy had on his inaugural drive down the nation's "main street. " He observed that this grand stretch of road-Pennsylvania Avenue in Washington, DC-was in a severe state of decline. Kennedy inspired architects to design an innovative tree-planting plan. Since there is little room for open soil along Pennsylvania Avenue, a method was devised to make space where tree roots could grow and thrive under the sidewalk. The sidewalk-rather than resting on compacted soil-was designed to float on subsurface water, and when trees need water it flows under the pavement.

Designing and building the Pennsylvania Avenue planting was difficult, but it provides an example for us to follow. Like many other good ideas, however, it has had some practical problems. The original plantings have grown twice as fast as trees planted in standard pits on nearby streets. But later plantings on Pennsylvania Avenue have not done so crews were not willing to follow the plans after the architects left the site. An unwillingness to break away from traditional ways and the rush to complete construction ended this renaissance.

New methods must continue to evolve for making space for trees as roads are constructed and reconstructed. Like the Pennsylvania Avenue example, tree planting must be part of the original plan and not an add-on after the project is complete. Trees are as much a part of the road a sidewalks, curbs, and drainage. well, apparently because construction

Including trees in a cityscape plan is not expensive when considered as part of the project. Bob Skiera, City Forester of Milwaukee, and his staff put together the accompanying pie chart to show the costs of an integrated construction package. Milwaukee is one of the few cities that includes the cost of "making space for trees" in the capital budget for road-building. Its lead is one that every city in the country should follow.

Compared to pavement, which takes 25 cents of every street-construction dollar, and sewer and water facilities, costing 52 cents, the two cents spent on trees is nominal. Even the grass growing beneath the street trees can be expected to receive more monetary attention.

Integrating tree planting and maintenance with roadway construction and repair is a simple idea that can have a profound long-range effect. Two cents is not a lot to invest in the re-greening of our communities, and can go a long way in helping planners architects, and engineers establish an ecological landscape in America's cities and towns of the future.
COPYRIGHT 1989 American Forests
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:Designing the Ecological City, part 2
Author:Urban, Jim
Publication:American Forests
Date:May 1, 1989
Previous Article:Window into a watershed: by taking the pulse of its lifeblood - the hydrologic system - scientists are learning to better gauge the health of the...
Next Article:The Acid Rain Controversy.

Related Articles
A tree named Fletcher.
Designing the ecological city.
The best way to plant trees.
The sustainable city.
Urban ecosystems: breakthroughs for city green.
Vestpocket forests ... in unusual places.
Cool Communities program blooms in Dade County.
Global problems, local solutions: measuring the value of the urban forest.
Roanoke rising.
So you want to plant a tree....

Terms of use | Copyright © 2016 Farlex, Inc. | Feedback | For webmasters