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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 Selector database?
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?

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.

  • 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 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.

Support provided by the US Forest Service - Urban and Community Forestry