Tree Planting Decisions

By Len Phillips

When an arborist or urban forester is asked by the mayor, city council, or tree advisory board to recommend the best way to preserve or plant new trees in the city, how do you respond? The following can be considered the best options for planting trees or installing sidewalks in conjunction with construction activities around a city sidewalk in various site and sidewalk conditions.

⊙ SITE – NEW CONSTRUCTION INCLUDING NEW SIDEWALKS AND NEW TREES       

Best Planting Option – During construction of new sidewalks, streets, utilities, and buildings – plant all the trees in groups 10 to 20 ft. (3 – 6m) apart or included shrub beds with excellent soil and growing conditions. Be sure to select trees that are not considered surface rooted trees. For lists of desirable trees, see the lists in the opening section of Topic #18. 

Second Best Planting Option – The use of soil cells is the next best method for ensuring there is enough suitable soil for a tree to develop into a mature specimen. Soil cells transfer a compacted foundation 3 ft.      (1 m) below grade to a surface suitable for a sidewalk, plaza, or light traffic roadway while allowing plenty of ideal soil for growing trees under the hard surface. The installation of root barriers to deflect roots downward should be included to prevent the tree's roots from causing damage to the surface pavement. The soil cells will also provide stormwater storage and reduce the frequency of manual watering of the newly planted trees.  See Topic #18 - Soil Cells section for more information.  

⊙ SITE – NEW SIDEWALKS AND PRESERVE EXISTING TREES       

Best Option – Carefully examine the condition of the trees and the existing roots to provide care, protection, and root pruning as necessary. Modify the sidewalk design and location to accommodate the tree and its roots. This option is often the result of sidewalk replacements caused by roots that have damaged the existing sidewalks. The best and most common solution is to raise the sidewalk higher which results in the roots being in deeper soil than before the new sidewalk is installed.  Include S-curves around the trees and pavement alternatives found in Topic #18 - Alternative Designs section.  

Second Best Option – Loosen and improve the soil including that which will serve as a base for pavement. New sidewalks should be built and suspended on pilings above the tree roots or on the top of soil cells installed under the new sidewalk depending on the existing root locations. See Topic #18 - Suspended Pavements section for more information.

⊙ SITE – NEW TREES AND PRESERVE EXISTING TREES     

Two Best Planting Options – Using an air tool, excavate as much soil as possible, under and around any existing roots, including the areas where the pavement is planned. Carefully examine the existing roots and prune if necessary. Follow the Stockholm Solution procedures of adding layers of large rock, washing soil into the voids between the rocks, adding another layer of rocks, and continuing this rock and soil process until the entire area that will become paved, is just below finished grade. 

An option would be to install CU Structural Soil which consists of mixing one inch (2.5 cm) stone and soil together and installing this mix into the area to be paved. After completing either option, add any new trees and root barriers as necessary, before finishing the job with the addition of a new pavement on the surface. See Topic #5 - Successful Planting section for more information.

⊙ SITE – NEW TREES AND PRESERVE EXISTING SIDEWALKS        

Best Planting Option – Consider relocating the planting site to the other side of the sidewalk, away from the curb and street, if the site is suitable and better soil is available. This assumes that the traffic volume and speed are not a pedestrian safety concern that might require trees as a safety barrier. This option may also require a local easement or agreement with the abutting land owner if the trees will be planted on private property. In cities where this option is approved, most of the maintenance of the tree is also transferred to the private land owner. See Topic #18 - Sidewalk Planning & Construction section for more information.

Second Best Planting Option – Excavate as much soil as possible for the tree planting site, to obtain 500 cu. ft. (14 cu meters) of space for installing good soil, include going as deep as 4 feet (125 cm). Punch, auger, or wash out holes from the bottom of the planting pit, under the sidewalk, toward good soil, if it is available beyond the sidewalk and let the tree's roots following the holes toward this better soil. Select a tree that is suitable for the amount of soil available. See Topic #18 Sidewalk Planning & Construction - Soil Volume section for more information. Plant the tree with excellent soil that is similar in texture to the existing soil around the planting pit. 

⊙ SITE – NO TREES AND NO SIDEWALKS  

Best Planting Option – All the planting options listed above are suitable. However, anticipate what the future might bring at this site and plant the appropriate tree at the appropriate site for the future. See Topic #18 for more information.

⊙ SITE – ROOT TREATMENT OPTIONS                                

Best Choice of Trees – Become familiar with how to plant bare root trees. They are the least costly to purchase and plant by contract and easiest to plant when using volunteers. Planting trees with bare roots in city soils will not delay their transition to different soils than what they were growing in at the nursery. See more information on planting bare root trees.  If there is a concern that the lack of a soil ball might cause the tree to blow over, this is easily remedied with any of several root stabilizing options that are currently available. See more information on root stabilization in our Archived Seminar #70 - Staking New Trees section.