Why Trees May Fail to Establish or Thrive (and what to do about it)

Landscape architects, architects, and engineers have many tools available to them in the design of healthy long-lived trees, and no single one of them can guarantee the establishment or long-term success of the trees. These tools need to be applied within a larger understanding of general principles of arboriculture, soil, and urban conditions in order to have an opportunity to yield healthy specimens years later.

While there are many potential problems that can impact tree growth, the following are the principle reasons why a tree may fail to establish or prosper after planting – and some recommendations for what you can do about them.

Establishment Period Problems

Poor quality root balls from the nursery

Root systems that are too deep in the root ball, have stem girdling, kinked roots, or roots that are not well distributed in the root ball will result in difficult establishment and long term growth problems. Container trees frequently have severely deformed root systems.

Solutions:

  • Change specifications to include requirements for quality root systems.
  • Tree root systems must be inspected at the nursery.
  • Plant trees bare root.

Too much water in soil

Over irrigation, soil with too much compost, and/or inadequate soil drainage slows or stops tree growth. Too much soil water can quickly kill a newly planted tree.

Solutions:

  • Specify soil and root ball moisture requirements.
  • Monitor soil and root ball moisture as part of construction observation.
  • Educate contractors and owners about over watering.

Too little water during establishment

The first year after planting is a critical time for proper watering. Trees that are too dry at any time during the harvesting, shipping, planting and initial establishment period will decline and be difficult to establish. Container tree potting medium is often a difficult water management problem.

Solutions:

  • Specify soil and root ball moisture requirements.
  • Monitor soil and root ball moisture as part of construction observation.
  • Educate contractors and owners about over watering.

Soil quality

Soil that has adequate water and nutrient holding capacity (2 to 4% organic matter) and conditions to support soil biology are important during both the establishment period and long-term success of the tree. Excess soil compaction reduces soil function and tree growth of even a healthy soil. A common problem with soil under pavement is that no way is provided to inspect or monitor the soil. It is critical to be able to inspect the soil and its moisture within the root ball, immediately outside the root ball and the soil between the trees. Very few projects provide for this function.

Solutions:

  • Educate yourself about soils.
  • Change specifications to require good soils. 
  • Reuse and improve existing soil wherever possible instead of imported soil.
  • Avoid installing screened soil.
  • Install soil inspection access in paving between trees and within the tree planting space.

Long-Term Growth Problems

Soil volume

Adequate soil volume is the primary limitation to long term tree size and health. Especially in urban areas where space is at a premium, street trees rarely have access to the amount of soil they need.

Solutions:

  • Understand the limitations of the existing soil.
  • Undertake a soil survey for each project.
  • Consider improvement to existing soil before deciding to import soil. 
  • For soil under paving, calculate soil volumes after the volume of any structural elements such as rock and or pavement support systems are subtracted (I recommend at least 1,000 cubic feet (28 cubic meters) of soil for long-term health and growth of trees).
  • Do not wrap installed soil volumes under pavement in filter cloth or other barriers that prevent roots from finding additional soil resources.

Water harvesting

Soil without water is not useful to the tree. Soils sealed off from surface water must be provided with a way to hydrate the soil. Harvesting rain is the most permanent method to hydrate soil. Pressurized irrigation in regions with rainfall, less than 30” per year, may be a reasonable alternative to harvesting water.

Solutions:

  • The best solution is to use pervious paving.
  • Capture surface water and distribute thru exfiltration pipes under the pavement.
  • Design systems only after fully understanding how water moves thru pipes and soil.
  • Assure that there is adequate capture of sediment before the piped system and that there are systems for cleanout of pipes.
  • Design irrigation systems for maintenance including replacement of drip lines under the pavement

Trunk flare constrictions

Trees planted with paving, tree grates, resin bonded paving or other structures that constrict the trunk as it grows can cause severe damage and even death.

Solutions:

  • Make spaces in the pavement that are larger then the expected trunk flare of the mature tree. A reasonable minimum width is 5 feet or at least 12 inches larger than the proposed root ball diameter.
  • Fill the tree space with planting, mulch, or loose gravel or stone dust.
  • Do not pave or use tree grates.
  • Resin stabilized gravel must be kept at least 12 inches from the face of the tree trunk.
  • Plant the tree with the trunk flare even to or higher than the adjacent paving.
  • Use root guides along the edge of the paving adjacent to the tree space to guide roots down rather then around the edges of the paving.

Tight spacing

As tree spacing is reduced, particularly in multiple rows of trees or trees in the shade of buildings, competition for light increases and tree growth slows. In extremely tight spacing trees will eventually die, shaded by trees that have even slightly greater access to light.

Solutions:

  • Space trees a no less than one half of the tree’s mature crown spread.
  • Use Dirr’s Manual for Woody Landscape Plants as a guide for mature crown spread.

Conclusion

The design, details, specifications, and construction observation of each tree must address each of these requirements in order to establish and prosper into a long-lived healthy tree. Many of these problems are embedded in the first design ideas and require that we simply design differently.

A good test of any design is to draw a quick section of the tree in the planting space at reasonable maturity. For example, an oak tree should have a trunk diameter in excess of 36 inches (915 mm) and a trunk flare diameter at least 80 inches (2032 mm). The tree crown diameter would be around 50 feet (15 meters) and extend about 60 feet (18 meters) in height. A sketch shows how all aspects of the design would function with the successful tree. In an ideal world, designers would use these sketches as part of their design submission to demonstrate that the design actually works, similar to how we draw the truck turning radius at the loading dock.

Sketch to demonstrate the predicted results of the design decisions.

Sketch to demonstrate the predicted results of the design decisions.

The new specifications and details for planting and planting soil published by the Urban Tree Foundation are a good beginning point for developing project documentation, but they cannot overcome fundamental design mistakes. Increasing construction observation time and reestablishing the importance of nursery inspections will also be critical to successful implementation of these concepts.

Thanks to the efforts of many researchers and practitioners, our understanding of common – and preventable – challenges to tree establishment and long-term success has grown substantially. We now need to execute on what we know. By following the recommendations described above in our project design, detailing and implementation, we can all help move the success of trees in cities forward. Try some of these things and let me know areas where you succeed and where you still struggle.

 

 James Urban, FASLA, is the author of “Up By Roots.”

Angie MoonCC BY-NC 2.0

2 comments

  1. Thank you for the great article. When reviewing streetscape projects for GDOT, the spaces in the pavement were required to be a minimum of 7 feet wide (rather than the 5 feet you mentioned to accommodate trunk flare), in order to avoid damage to the curb and paving sub-base at the edge of the road. It was a difficult policy to enforce, but I like to think we saved a lot of urban street trees in the process.

    Also, smaller tree pits caused the trees to flare and uproot the adjacent pavement, so the pavement was not longer ADA compliant.

  2. Dave: I totally agree with your comment, and my sketch showing a 5 foot tree space also shows that the result is a constricted trunk base and lifting. The sketch is to show the negative results of design decisions. I chose 5 feet for the sketch because most trees are planted in 4 foot openings and I wanted the example to be closer to what is being installed rather than the optimum. Maybe I needed to be clearer and thank you for writing about it.
    I endorse any tree space size that is larger than what is currently being designed. 7 feet is great but as you note it is tough to sell and in some cases tough to actually implement in narrow sidewalks. BTW I have seen lifting even in a 10 foot wide space so I think there is actually no space that is wide enough in a sidewalk situation to assure no lifting from large mature tree roots. Root guides along the edge do help reduce lifting and increase the time until lifting occurs. Ideas like Silva Cells are going to let us grow much larger trees in cities so a by product will be more sidewalk conflicts if we do not consider all the realities of tree space design for these larger structures. Unfortunately I see designers using Silva Cells without making the tree spaces larger. Plant fewer trees, at wider spacing, in larger tree spaces and we will have a better urban forest!

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