What is the Virginia Street Tree Selector?
The street tree selector is designed to serve as a resource and forum for street trees. Trees in downtown areas are under intense pressures from pavement, buildings, and people. Many factors restrict tree choices in these sites: reduced rooting areas, extreme pHs, clearance requirements, etc. Finding a variety of trees that will perform well in these situations can be difficult. If you are planning tree plantings for a streetscape, a median, a parking lot, or main street, we hope you will be able to find some suggestions using the search functions provided here. The site is designed for Virginia communities, but will also be useful to the Mid-Atlantic region and beyond.
A Note about our Photos
In the tree selector, our aim is to show street
trees as they really are--the successes and the failures. We encourage
reviewers to post pictures and comment on the success of the planting
shown. See our photo guidelines
for more.
Are you an Urban Forester, Municipal Arborist, Town Horticulturist or other professional?
We encourage you to share your favorite trees or your biggest planting failures
via the treeselector. Post a review and picture. If any tree isn't included
(to see the complete list, just click "Find a Match" with
no qualifications) send us your recommendation at urbanforestry@vt.edu. Through your participation,
the Virginia Street Tree Selector will continue to grow and improve.
Resources
Many sources have been consulted in characterizing trees in the Selector. Among these are:
Appleton, B. L. and L. T. Chaplin. 2001. The New York/Mid-Atlantic Gardener’s Book of Lists. Taylor Trade Publishing, Dallas, TX.
Dirr, M 1998. Manual of Woody Landscape Plants, 5th ed. Stipes Publishing, Champaign, Illinois.
Bassuk, N. L., D. F. Curtis, BZ Marranca, and B. Neal. 2003. Recommended Urban Trees: Site Assessment and Tree Selection for Stress Tolerance, Urban Horticulture Institute, Cornell University, Ithaca NY.
Gerhold, H. D., W. N. Wandell, and N. L. Lacasse,
Eds. 1993. Street Tree Factsheets. PennState College of Agricultural Sciences,
State College, PA.
Gilman's Tree Selector. Details on the culture of many woody plants are available at this web site by Dr. Edward Gilman of the University of Florida.
Submission Policy
By providing content to our site, you have granted Virginia Polytechnic Institute
& State University (VPI & SU) unrestricted use of that material and
grant permission to post your review with your name, title, city and state.
Your E-mail address will not be revealed to other entities by VPI & SU without
your permission. We reserve the right to modify, reproduce and distribute materials
posted to our services, both internally and for non-commercial use.
We have the right, but not the obligation, to remove or edit any content submitted to this site. Information that you place on the Street Tree Selector must be your own original content. You are responsible for assuring that any material (text and images) you provide does not violate, plagiarize or infringe upon the right of any third party, including copyright, trademark or proprietary rights. If non-original content is included in your posting, you must obtain permission from the content owner and provide due credit. Information contributed represents the views of the person submitting the review and is not necessarily endorsed by us.
Contributors and Resources
We are very appreciative to the long list of contributors and resources we have relied upon to create this site. We gratefully acknowledge the assistance of the USDA Forest Service through an Urban & Community Forestry Assistance Grant in creating this web site.
The following people have provided photographs, content, guidance, or resource material concerning the Virginia Street Tree Selector and we gratefully acknowledge their participation. –Susan Day, Research Assistant Professor, Urban Forestry and Andrew Meeks, Programmer.
Bonnie Appleton
J. Roger Harris
Alex X. Niemiera
John Peterson
John Seiler
We gratefully acknowledge the sponsorship of the Virginia Department of Forestry through its Urban & Community Forestry Grants program for the construction of the Virginia Street Tree Selector.
Frequently Asked Questions about the Virginia Urban Street Tree Selector
Q. How
does the Tree Selector use the information about the size of the planting
site?
A. The Tree Selector uses a variety of data to suggest
trees that will grow in the soil volume provided by your planting site.
The size of the planting site, the soil compaction level, the tree’s
ability to escape planting pits, and the presence of structural soils
or other break-out systems all affect the size tree that can be grown
in a given site. The Tree Selector uses these data and the expected ultimate
dbh (trunk diameter at 4.5 feet above ground) of each tree species to
create a list of suitable trees.
It is important to understand that the expected ultimate dbh attributed to each tree is NOT the genetic potential of that tree. For example, a Ginkgo biloba could reach 4 or 5 feet dbh if growing conditions were ideal. However, as a downtown street tree (the focus of the Tree Selector), 24 inches dbh would be a reasonable ultimate size—not necessarily the full genetic potential, but representative of a long-lived and serviceable street tree.
Q.
What is “usable soil volume”?
A. Usable soil volume describes the amount of
soil available for tree root growth. For example, if a tree is planted
on a severely compacted clay soil, the usable soil volume will be only
the soil disturbed during installation or a few inches on the surface
loosened by mulching—perhaps less than 2 or 3 cu. ft. Tree roots
grow primarily in the top 2 feet of soil, and most grow very near the
surface. For this reason soil below 2 feet would not be considered in
soil volume calculations in most cases. For example, if a tree is planted
in a sidewalk cutout that is 4 x 4 feet and the soil is uncompacted, then
the usable soil volume could be considered to be 4 feet x 4 feet x 2 feet
deep, or 32 cu. ft.
Q.
How much soil volume do trees need?
A. Large shade trees in downtown settings rarely, if
ever, have sufficient soil volume to grow to their full potential size.
Many models for predicting
the volume of soil required have been proposed. In our region, a useful
rule of thumb is that each inch of dbh (trunk diameter at 4.5’ above
the ground) requires about 20-25 ft2 of open ground with uncompacted soil.
However, you will find that this amount of soil is rarely provided. Trees
do survive, but do not reach their expected size. A tree may establish
and grow normally for a few years. Then, when there is no longer enough
soil for the tree’s increasing size, growth dramatically slows and
the tree declines prematurely. Some trees are able to overcome the situation
by rooting under sidewalks and through cracks to access adjacent lawn
areas.
Q. What are some
of the models for calculating soil volume and how do they differ?
A. There are several available soil volume models (listed
below). As an example, let’s look at a 24” dbh Zelkova
serrata with a crown spread of 50’. Using the Lindsey &
Bassuk model, this tree would require about 2200 cu. ft. in a typical
soil in Blacksburg, Virginia. The Urban et al. model would predict
about 1500 cu. ft. would be required. The Natural Forest method based
on upland hardwoods in the Eastern U.S., would predict that between 2700
and 4100 cu. ft. would be required, depending on soil quality. The rule
of thumb mentioned above (each inch of dbh requires about 20-25 ft2 of
open ground with uncompacted soil) would estimate 960 to 1200 cu. ft.
To some extent, results will depend upon the species, as some species,
such as Zelkova, are better able to exploit soil resources under pavement
or in compacted areas. The larger soil volumes will support a healthier,
more vigorous tree, but these volumes may be unattainable in restricted
downtown areas.
Q. Why do
I get no trees when I put the size of my planting site in my search?
A. Your planting site may be too small or your soil conditions
so poor, that it will not support tree growth for long. You can either
modify your site to improve it, or plant trees that will have a severely
shortened life span. If you cannot modify the site, enter criteria for
a somewhat larger planting site. You can plant these trees in your site,
but you must recognize that their ultimate size and lifespan may be severely
curtailed.
Q. What is
soil bulk density?
A. Bulk density is a measure of soil compaction. It is
the oven-dried weight of an amount of soil divided by the volume it occupied
in the ground. Bulk density is usually expressed in grams per cubic centimeter
(g/cm3). In mineral soils (i.e. not organic-based potting media), bulk
density is typically between 1 and 2g/cm3. Bulk density is usually measured
by taking undisturbed core samples of known volume and drying them in
an oven at 100-105 C. The net dried soil weight is then divided by the
volume. You can also measure bulk density by digging a small hole and
collecting the soil. Then place a plastic bag in the hole and fill it
with water. Measure how much water it takes and this will give you the
volume. Then dry the soil in an oven set at roughly 100 C (180 F) and
again divide the weight by the volume. When soil texture is also known,
bulk density provides an accurate assessment of soil compaction level.
Q. Why
does the Street Tree Selector ask for the soil texture at my site?
A. The soil texture influences the amount of water a
soil can hold, soil drainage properties, and how soil responds when compacted.
Soil texture is the proportions of sand, silt, and clay in a soil and
can be determined during a site analysis.
The Street Tree Selector uses this information to exclude certain trees
that will not grow well in your soil. In addition the Street Tree Selector
indicates (via a mouse rollover) the bulk density
of a particular soil at each of the compaction levels to help you diagnose
the compaction level of your soil. For example, if you measured the bulk
density of your soil to be 1.4 g/cm3, this would be a compacted soil if
it were a clay loam, and an uncompacted soil if it were a sand. Your choice
of soil texture will have only a small effect on trees selected—so
if in doubt, just choose “any soil texture.”
Q. Why can’t
I find some trees in the Tree Selector database?
A. The Tree Selector is designed to provide you with
a list of suggested trees for highly built environments. Trees that are
not recommended for such sites because of severe disease problems, high
potential for invasiveness, or other problematic characteristics, are
not included. Trees that are unlikely to fit the requirements of such
sites because of there large size or form are likewise not included. If
you have a tree that you would like to recommend for inclusion, please
contact us.
Q. I live outside
Virginia, can I use the tree selections from your site?
A. The Tree Selector is designed for Virginia. It can
be used to generate tree lists for other parts of the country, but we
recommend that you thoroughly research all proposed trees. Some of them
may not be suitable for your area either because of differences in climate,
or because of regional disease or insect issues. Furthermore, the potential
for exotic trees to invade surrounding natural areas can differ by climate.
Q. What
is the “urban zone”?
A. The urban zone is just the term we use to ask you
which physiographic region of Virginia your tree will be planted in. The
Mountain, Piedmont, and Coastal Plain regions vary widely in climate,
and species should be selected that will do well in these varied climates.
On the Coastal Plain, heat tolerance should be considered and in the mountains,
cold hardiness is very important.
As always, we recommend thoroughly researching unfamiliar trees proposed
by the Tree Selector by referring to a reliable source such as Michael
Dirr’s Manual of Woody Landscape Plants. A tree is a long
term investment—a careful choice has a much better chance of providing
long-term satisfaction.
Q. What
is structural soil?
A. Structural soil refers to a group of soil mixes that are designed to
be both load bearing AND to support tree growth. Because they are load
bearing, they can be used under pavement, including streets. They typically
function by using large gravel or similar material that form a lattice
work that provides the support needed to hold up pavement. A good topsoil
is mixed in and fills the voids between the gravel, providing uncompacted
growing space, moisture and nutrients for roots. Structural soils can
greatly increase rooting area for urban trees. Although many municipalities
plant trees directly in structural soils, they can be confined to the
underpavement areas with ordinary soils used in the open “tree pit”
areas.
Many structural soils (such as those made with limestone gravel or Carolina Stalite) have high pHs and appropriate trees need to be selected. There are several types of structural soil: CUSoil™ [leaving VT] was developed at Cornell University and can be made with local materials by a licensed contractor. Carolina Stalite [leaving VT] is an expanded stone material that can be mixed with topsoil to produce a structural soil. Other types of structural soils are also available in some regions.
Q. What happens when tree roots get very
large in structural soil—will they buckle sidewalks?
A. Structural soils appear to have the added benefit of allowing tree
roots to grow deeper in the soil profile—so they are less likely
to exploit the area of low resistance between compacted soil and pavement
that exists in some sidewalks. As tree roots expand, the gravel has been
observed to become embedded in the roots. Root morphology does change
somewhat as roots grow around the gravel lattice—but root expansion
is ultimately not impeded. Trees can be planted directly in structural
soil with pavers coming up to the tree trunk. However, by planting in
ordinary soil with structural soil under adjacent pavement, trees may
establish better and buttress roots may be better accommodated.
Tree species vary considerably in their minimum requirements for rooting volume, pH, etc. Without knowledge of the planting site and the silvics of a variety of species, communities often choose broadly tolerant and overused species such as Bradford pear. It is good practice to match species with site and diversify the species composition of a community.
Making Site Improvements to Expand Species Choices
Soil conditions are often the most limiting factor
for tree growth in downtown areas. In order to use the largest selection
of species and provide the greatest potential canopy cover, the ability
of the site to support tree growth must be maximized. Common site limitations
and techniques for addressing these limitations need to be considered.
After improving site conditions, a search in the street tree selector
should provide you with many more species choices. Click on one of these
buttons for specific information in this process.
Soil Volume
This red maple has reached its maximum size for this planting site. Although the tree survives, it will never reach its genetic size potential. The tree has been growing on this site for 10 years. It is 6 inches in diameter and has yearly shoot growth increments of less than 12 inches. These trees look reasonably healthy in early spring, as in this picture, but show premature fall color and dieback by the end of the summer.
In order to support an adequate root system that will see the tree through long hot summers and cold, especially dry winters, soil rooting volume must be maximized. If you have the opportunity to influence the design of tree spaces, much rooting space can be gained. Techniques for maximizing rooting volume include:
If you must work with the existing design, rooting space can still be maximized within the existing space by increasing the usable soil volume. Compacted soil reduces usable rooting volume by limiting root growth as the soil hardens when dry or becomes suffocating when excessively wet. Even trees in open lawns display the characteristics of the red maple in the photo to the right when soil is severely compacted. An uncompacted soil with good structure and organic matter content will allow more root exploration and have a more desirable mix of plant-available water and air—essentially providing more rooting resources in the same space! Pre-planting techniques include:
Soil Compaction
Reducing soil compaction will have the added
benefit of improving water infiltration into the soil profile—providing
more water to your trees and less to the supply of urban runoff.
Reducing Compaction Example
Problem: A severely compacted median in a wide roadway is to
be planted with large shade trees as part of the entrance into downtown.
The median is 12 ft. wide. Its soil is primarily a clayey subsoil, heavily
compacted by road machinery during construction. Even in this wide median,
usable rooting volume for a tree is about 300 ft3.
Solution: Subsoil tillage loosens lower layers. Two inches of municipal leaf compost is spread over the surface and tilled in thoroughly with a rototiller to a depth of 6-8 inches. During tilling, debris is removed. Soil with excessive stones or gravel is screened. Soil is now relatively uncompacted, and usable tree rooting volume within the median has been increased to about 1200 ft3.
This is a one-time activity that will pay dividends for many years.
Soil Alkalinity and Acidity
Most trees grow best in neutral to somewhat acid soils (pH 7 and below).
Some, such as pin oak and willow oak (Quercus palustris and Q.
phellos ) will decline rapidly when soil pHs are above 7. In downtown
settings, where concrete is abundant and limestone gravel may be mixed
in the soil or used as a base layer for pavement, soil pH can be very
high (alkaline). These high pHs limit nutrient availability to plants
and often result in chlorotic (pale green or yellow) leaves. Fortunately,
there are some species that will tolerate these higher pHs and these are
often selected as street trees.
This tree never had a chance: Quercus phellos won't tolerate the high pH at this site.
Example
You may want to consider adjusting the pH of
the soil at your site in some situations. Alkaline soil (high pH) is the most common problem with street
trees. Generally, the more clay in a soil, and the more widespread
the sources of alkalinity, the more difficult it is to lower pH... and it is likely to return to its present level in the long
term. Consider a clayey soil with a pH of 7.2 that has limestone gravel from a nearby sidewalk mixed in. To lower that pH we would have to remove the limestone gravel and till in agricultural sulfer according to the table below (pounds per 1000 square feet) (source: Virginia Cooperative Extension). Note that microbial activity is required, so warm and moist conditions are required for fast action.
| Existing pH | Desired pH | Clay | Loam | Sandy Loam |
|---|---|---|---|---|
| 8.0 | 7.0 | 32 | 20 | 8 |
| 8.0 | 6.5 | 47 | 29 | 12 |
| 7.5 | 7.0 | 16 | 10 | 4 |
| 7.5 | 6.5 | 32 | 20 | 8 |
| 7.0 | 6.5 | 16 | 10 | 4 |
| 7.0 | 6.0 | 32 | 20 | 8 |
A thorough site evaluation should tell you how well a proposed planting site will be able to support street trees and what specific site limitations are. This Site Evaluation Form will guide you through this process. After the site evaluation is complete, select appropriate species and cultivars. The specific site for each tree should be considered to make sure a plant selection is made that will thrive in that spot.
Our aim is to provide a useful tool for greening Virginia’s communities. If you have suggestions or would like to report an error in the database—please give us your feedback.
