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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.
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:
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.
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.
You may want to consider adjusting the pH of the soil at your site in some situations. Typically the need is to lower pH as overly alkaline soil (high pH) is the most common problem with street trees. First, you must have determined the current site soil pH and develop a feel for the soil texture at your site and the sources of alkalinity (see site analysis page for information on gathering this data). The more clay in a soil, and the more widespread the sources of alkalinity, the more difficult it is to lower pH. Even if pH is lowered, it is likely to return to its present level in the long term. For this reason pH lowering is most easily achieved when some or all of the source of alkalinity can be removed, the clay content is relatively low and the desired change in pH level is not too great. In the example below, pH can likely be lowered at least temporarily with a resultant improvement in tree health. Tree species that will tolerate a pH of 7.0 or higher, however, should still be selected, as the pH will likely rise over time.
If the sources of alkalinity cannot be removed—for example if a limestone-based structural soil is in use, or if limestone gravel is mixed into the soil and cannot be removed, then attempts to adjust pH are likely a waste of resources.
Site improvement in urban areas can play an important role in increasing both the species diversity and the canopy cover of our urban forests.