Big Tree, Small Hole

By Buck Abbey, ASLA

In almost every instance, it was noted that communities were sizing the planting space but not regulating what size tree should go there. Often large trees?EUR??,,????'??+oaks, spruce, redwood and magnolias?EUR??,,????'??+would be planted in the same size planting area as ornamental cherry, crab apple, sabal palm or crape myrtle.

Tree Plantings by Code

The planting of trees within vehicular use areas or parking lots according to most landscape codes is based upon selective spacing, linear feet, square footage, parking space tree ratios or distance to a shade tree. Most landscape codes in American communities regulate the size of tree planting spaces.

In Ft. Pierce, Fla., for instance, a minimum size area of 100 square feet is essential for preservation of trees. The planting area size is established based upon ?EUR??,,????'??species, age, size, condition of the tree, soil condition, topography, means of protective barrier proposed,?EUR??,,????'?? for the purpose of protecting the roots and trunk of a tree during or after construction.

From the Salisbury North Carolina Design Guidelines Manual, this area is portrayed as a critical root zone (CRZ) area. It is calculated as ?EUR??,,????'??one foot of radial distance for every inch of tree DBH (diameter at breast height), with a minimum size of eight feet.?EUR??,,????'?? In McAllen, Texas this minimum planting area, called a ?EUR??,,????'??plantable area,?EUR??,,????'?? is ?EUR??,,????'??40 square-feet with a minimum dimension of 36 inches.?EUR??,,????'?? In Chicago, this minimum planting space within vehicular use areas is 125 square feet with a minimum dimension of 10 feet. Within ?EUR??,,????'??parkway planters?EUR??,,????'?? (tree-planting spaces along the edge of public street right-of-ways) this dimension is reduced to 36 inches.

?EUR??,,????'?? Setting standards adds precision and predictability to the regulatory process.?EUR??,,????'???EUR??,,????'??+Christopher Deurksen, Tree Conservation Ordinance, 1993

It is interesting to note tree planting in parking lots is not always based upon the size of the tree or its requirements for growing successfully. Most communities base their tree planting area size on some number related to automobile circulation, not based on the best interest of the trees.

This insight was gained several years ago while assisting a community in drafting a local landscape code. During the discovery process of looking for innovative parking lot solutions across the country, it became apparent that parking lot design standards or technical requirements were drafted for cars, not trees. This was reinforced by visits to designed parking lots in several states where the fact was observed over and over again in the field. In almost every instance, it was noted that communities were sizing the planting space but not regulating what size tree should go in to that planter. Often large trees likes oaks, spruce, redwood and magnolias would be planted in the same size planting area as ornamental cherry, crab apple, sabal palm or crape myrtle.

Upon inspection of the written codes from several communities from across the nation it became obvious that parking lot tree planting areas were sized to fit into the parking lot grid, not sized for optimum growth of the trees or even the minimum amount of space needed for the size, species or type of planting to be installed. Landscape architects have all seen examples of big trees planted in very small holes.

Volusia County, Florida?EUR??,,????'???s landscape code is a typical example of how planting areas are sized for cars and not trees. In this southeast Florida community, interior landscaped areas are dispersed within the parking area to define aisles and limit rows of parking to a maximum of 200 feet or 20 parking spaces. Landscaped peninsulas at the end of each parking row have a ?EUR??,,????'??minimum area of 175 square feet?EUR??,,????'?? with no width less than 10 feet and a length of 17.5 feet if it abuts one parking space, or 35 feet if it abuts end to end parking spaces. Interior landscaped areas must total 10 percent of the total off-street parking area.?EUR??,,????'?? The code requires that interior landscaped areas, other than those at row ends, shall have a minimum area of 100 square feet with no dimension less than 10 feet. Two feet of these landscaped areas may be part of the required depth of each abutting parking space, provided wheel stops or curbs are used to protect the planting area.

This effectively reduces the size of the planting area by two feet by allowing overhang into the planting area.

The landscaped area must include at least one tree, but no specific tree size is required. And this is the crux of the problem.

And of course, this code provides an escape clause based on cars and not trees. The Volusia code allows planting reductions if the strict application of the interior parking lot planting subsection seriously limits the function of the parking area.

No mention is made in this code of islands, medians or parking lot screens, spacing requirements or distance to shade, which are all commonly found in other codes. There is no sliding scale parking-space-to-tree ratio as used in some communities to require a greater number of trees in lots with greater number of cars, or to set tree density standards or canopy coverage requirements. These standards are found in many other landscape codes to regulate the size of minimum planting areas.

The Beavercreek, Ohio zoning code for parking lots allows up to 15 spaces between islands.

Tree Plantings Areas by Design

The problem with the Volusia code is that the designer may use any size tree within the 175 square foot planter. A good idea for community landscape codes to size the tree planting areas to the size of tree to be planted. Since tree size and growth is dependent upon available root space, it would make sense to have at least three different size planting spaces to regulate the planting of trees within paved areas of a development site.

Some communities, such as Covington, Louisiana size trees by class. Class A, Class B and Class C are commonly used labels to describe what wound normally be referred to as overstory trees, understory trees and small ornamental trees. The latter would include large, flowering, fruiting and multi-stem shrubs growing 10 to 15 feet tall. Therefore, it would make sense for communities to size planting areas to regulate the size of trees planted within the minimum size planting spaces. Landscape code administrators could do this by specifying several planting area size standards for planters, islands, medians and peninsulas that would be appropriate for Class A, B and C trees. Think of them as class A planters for Class A trees.

Volusia County, Florida?EUR??,,????'???s landscape code is a typical example of how planting areas are sized for cars and not trees. No mention is made in this code of islands, medians or parking lot screens, spacing requirements or distance to shade, which are all commonly found in other codes.

Scaleable Tree Planters

The largest planters, class A, should have a minimum area of 17?EUR??,,????'???x15?EUR??,,????'??? (255 sq. ft.) or 17?EUR??,,????'???x20?EUR??,,????'??? (340 sq. ft.). The use of class A trees within the cramped spaces of a parking lot is questionable but it is seen in many communities. Having a larger planter than is commonly specified would assist communities who insist upon large shade trees within in parking lots. A 340 sq. ft. space would grow the Class A tree well.

Class B planters are a little smaller. Planters of this type would allow the use of medium sized trees within parking areas, which are the class of tree most appropriate for parking lots, plazas, courtyards and other paved site areas. Class B planters would be 17?EUR??,,????'???x10 (175 sq. ft.), the same as Volusia?EUR??,,????'???s and very appropriate for parking lot scale trees.

Class C planters might be used for planting ornamental trees and small tree-like shrubs. Class C planters could be as small as 5?EUR??,,????'???x17?EUR??,,????'??? (85 sq. ft.) and could add much more variety to the parking lot planting palette.

In fact, the use of large shrubs as small trees such as wax myrtle, Russian olive, lilacs, ligustrum, holly, titi, buckeye and vitex is often overlooked by designers. Their smaller fibrous, root systems and more diminutive size and compact structure can make ideal parking lot planting solutions if the objective is not canopy coverage.

Most communities in the U.S. base their tree planting area size on some number related to automobile circulation, which is not always in the best interest of the trees. This parking area in Obersendling, Munich, Germany looks like it gives priority to the trees.

Preserved Trees and Clustered Plantings

There are other types of parking lot planters that could also be used. These planters can be used for other functions in addition to planting.

Class P planters (tree preservation planters) would be used to preserve exiting trees within parking lots. These planters would be sized based upon the size and species of the tree to be preserved. Most landscape codes set standards for tree protection areas based upon tree caliper, tree size, crown spread or diameter to the dripline. Class P planters in the form of islands within parking lots should have a minimum size of 30’x40’ (1,200 sq. ft.), perhaps larger in the case of trees the size of Quercus virginana, Fagus grandifolia, Ficus benghalensis or Magnolia grandiflora or other large growing trees.

Let It Snow

Landscape codes rarely mention snow, although snow in parking lots is a winter fact of life in states from Alaska to Massachusetts. Therefore, special islands called Class S planters (snow grove islands) could be provided within northern parking lots that can serve a duel purpose. They can be planted as tree groves for summer and for the temporary storage of non-salted snow in winter. In milder climates, these same areas can be used to mitigate storm water runoff.

Communities should take a careful look at the relationship between the size of tree planted and the minimum space that is required for its successful growth and health.

D.G. ?EUR??,,????'??Buck?EUR??,,????'?? Abbey, Associate Professor of Landscape Architecture at Louisiana State University, is LASN?EUR??,,????'???s Associate Editor for Ordinances.