Frequently Asked Questions
about the Virginia Urban Street Tree Selector
How does the Tree Selector
use the information about the size of the planting site?
What is “usable soil volume”?
How much soil volume do trees need?
What are some of the models
for calculating soil volume and how do they differ?
Why do I get no trees when I put the size of
my planting site in my search?
What is soil bulk density?
Why does the Street Tree Selector ask for
the soil texture at my site?
Why can’t I find some trees in the Tree
I live outside Virginia, can I use the tree selections
from your site?
What is the “urban zone”?
What is structural soil?
What happens when tree roots get very large in structural
soil—will they buckle sidewalks?
does the Tree Selector use the information about the size of the planting
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.
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.
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
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
- Lindsey & Bassuk. This model uses
evapotranspirative demand, rainfall data, soil water holding capacity
and leaf area index to calculate the amount of soil needed in a given
climate to support the water demands of a tree of a given size. See
Lindsey, P. and N. L. Bassuk. 1992. Redesigning the urban forest from
the ground below: A new approach to specifying adequate soil volumes
for street trees. Journal of Arboriculture 24 (3): 25-39.
- Urban et al. This model is
for the Eastern U.S. and is based upon the tree’s canopy spread
and dbh. It was developed using data from a number of authors and the
experience of this urban tree expert. See Urban, J. 1992. Bringing
order to the technical dysfunction within the urban forest. Journal
of Arboriculture 18(2):85-90.
- Natural Forest. This method is derived
from stocking charts for upland hardwoods in the Eastern U.S. (USDA
Forest Service Ag. Handbook 355). We assume that density-induced stress
begins at approximately 100 ft2 of basal area per acre.
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
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.
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
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.
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
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”
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.