City Tree Planting 2017

By Len Phillips

The way we should be planting trees in our cities today has changed. Thanks to a lot of researchers, they have determined that there are better ways to plant city trees in the typical tree pit of a city sidewalk. Gone is the digging of a 3 feet by 3 feet by 2 feet (1x1x0.6 meter) hole between the sidewalk and the street curb or a missing section of sidewalk. Also gone are the above ground stakes to hold the tree from tipping over. Here are the results of the latest research efforts to grow trees in our cities. Your comments are welcomed.

The Best Way

The best way to grow trees in the city consists of using soil cells or structural soils for ensuring there is enough suitable soil for a tree to develop into a mature, healthy, specimen. Soil cells transfer a surface suitable for a sidewalk, plaza, or light traffic roadway to a compacted foundation 3 ft. (1 m) below grade while allowing plenty of ideal soil for growing tree roots in between and within the soil cells. The soil cells will also provide stormwater storage which reduces the frequency of manual watering of the newly planted trees. However, soil cells are expensive and are usually reserved for major street and sidewalk reconstruction projects.

A less expensive option to the soil cell 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 a deep trench, over which will be a new pavement. The planting pit should be at least 3 feet (1 m) deep and as wide as possible. CU Structural Soil is also reserved for major street and sidewalk reconstruction projects. 

The least expensive alternative is to go with the Stockholm Solution. This procedure requires the installation of layers of large rock where any pavement will be. The planting pit should also be at least 3 feet (1 m) deep and as wide as possible. The rock is compacted and an ideal planting soil is washed into the voids between the rocks. This process is repeated by adding 10 – 12 inch (26-30 cm) layers of rocks and soil until the entire area that will become paved, is just below the finished grade. In Stockholm, this procedure is used for street and sidewalk reconstruction projects as well as for improving the growing conditions around existing trees that are suffering from compacted soil or the lack of sufficient growing area for the roots.  This process should not be used where ample, uncompacted soil is available within the tree's future root zone. 

After completing any of these options, plant the trees and install root barriers as necessary to deflect roots downward to prevent the tree's roots from causing damage to the surface pavement. The planting should be done before finishing the job with the addition of a new pavement at the surface.

Sidewalk Tree Planting

Before planting a replacement tree in a sidewalk cutout or tree lawn next to the street, 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 option may require a local easement or agreement with the abutting land owner if the tree 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 more information

If the tree has to be planted in the sidewalk planting pit, 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). If possible dig a trench for the roots to share, between all the trees being planted. If this is not possible, punch, auger, or wash out small diameter 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 these holes toward the better soil. After considering these available options, select a tree that is suitable for the amount of soil available. Plant the tree with excellent soil that is similar in texture to the existing soil around the planting pit. Click Here to see more information and scroll down to the section dealing with soil volume.

The Best Choice of Trees

The trees to be selected should be trees that will tolerate heat as global warming continues to harm heat sensitive 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.

Staking

Recent research has indicated that staking a tree causes more harm to the tree than using root stabilizers. If there is a concern that the lack of a soil ball might cause the tree to blow over or be knocked over by vandals, this is easily remedied with any of several root stabilizing options, such as tree staples, that are currently available. See more information on root stabilization.

Surface Treatment of the Planting Pit

After the tree is planted and roots are stablized if necessary, the best surface treatment is to consider using one of the two best living mulches.  The living mulch is intended to beautify and replace the need for a tree grate or conventional mulch by using plants to provide weed suppression, control of the soil temperature, and a visual means to discourage people from walking close to the tree and causing soil compaction.  Living mulches in conjunction with a short term wood chip mulch can reduce water runoff and erosion, and filter surface water runoff for use by the tree.  Living mulches have also been shown to increase the population of organisms which enhance the quality of the soil which then benefits the tree.  

If this option is not acceptble to the community leaders, the next best option is to go with porous pavement. This is a fully permeable paving product which allows water to freely drain through the surface. The porous pavement's non-skid, 50% rubber content and 50% stone aggregate surface along with a hard urethane binder ensures good traction even when wet, lessening the chance of slip and fall accidents.  In addition, the rubber content allows the product to flex with trunk growth, or with any roots that come to the surface.  This flexibility resists cracking and heaving commonly found with  concrete sidewalks.  The porous and flexible rubber content prevents soil compaction but allows air and surface water penetration, which encourages the tree roots to grow deeply.

Watering the Tree

In this author's opinion the best watering method for the first growing season after planting, is to use the large volume water bags. They are easily and quickily installed and refilled with a garden hose or from a water truck, but take several days to leak the water out and over the rootball before they need to be refilled. Plus they can be reused for a few years.

The amount of water to be applied should follow this chart:

1. daily watering for the first one to two weeks, applying approximately 1.5 gallons per caliper inch (1.5 liters per caliper centimeter) per watering,

2. after two weeks, water the trees every two to three days for the next two or three months, with approximately 1.5 gallons per caliper inch (1.5 liters per caliper cm)

3. after 2-3 months, weekly watering is necessary for the remainder of the season or until the tree is established.

4. after the first year, if watering is necessary, it should be 1 gallon of water per inch of caliper. This equates to 1 inch of rainfall per week.

Newly transplanted trees are absorbing water from a diminished rooting area, so apply all the water to the root ball area. Roots must generate and grow into surrounding soils before a larger soil volume can be tapped for moisture. When tree roots have spread into the surrounding soil, they are considered established.

All of the suggestions provided here represent the latest research results for growing trees in our cities. As global warming contines to break temperature records every year, planting trees in our cities becomes more important and more challenging every year.

Your comments are welcomed.