How to Design for Urban Trees that Thrive (Part 2)

Part 2 of our three-part series on creating thriving urban forests. (Click here to read Part One)

In Part One, we explored the pre-planning essentials for successful tree growth. Now, let’s discuss the practical aspects of urban tree planting, focusing on design techniques and overcoming urban constraints.

We’ll look at:

• How to determine the needed soil volume.
• What tree planting options are available to you, and the pros and cons of each.
• Various layout options – making your space work for you.
• Working with space constraints & utility conflicts.

Understanding the Needs of Trees

Trees are forest creatures, not urban sculptures.

Yet too often, we install them as if they were inanimate objects rather than living organisms. Planting a tree in a cramped 4’x4′ pit with compacted soil is like keeping a whale in a swimming pool—it simply cannot thrive. This mindset contributes significantly to current urban tree mortality rates and wastes the resources invested in tree installation.

While we can’t perfectly replicate forest conditions in urban environments, we can come close with proper planning and execution. The key lies in understanding soil requirements and choosing the right planting medium.

 

Calculating Soil Volume Requirements

The soil volume your tree needs depends on several factors:

• Species selection
• Environmental conditions
• Desired mature size
• Design intent

For example, a small ornamental dogwood requires far less soil than a large live oak destined to be a plaza’s focal point.

As a general guideline, we recommend 300-500 cubic feet of soil per tree. When trees share soil volume, this requirement can be reduced by 20%. When in doubt, aim for a higher soil volume. The more soil trees have, the quicker they will grow and longer they will last.

 

Choosing Your Planting Medium

Your choice of planting medium is crucial for maximizing the limited space in urban areas. Different methods contain varying amounts of structural support, which can significantly reduce the space available for root growth. The most effective way to achieve desired soil volume is to choose a planting medium with a high usable soil volume.

Let’s explore the four main approaches to urban tree planting, each with distinct advantages and limitations:

1. Compacted Soil

While this is the cheapest and fastest method, it’s also the least effective. Soil compacted to support pavement (95-97% proctor density) becomes hostile to root growth, which requires less than 85% compaction. The result? Stunted growth, poor drainage, and unstable trees with surface roots that damage pavement.

2. Open Planting Areas

These provide ideal growing conditions with 100% uncompacted soil volume and natural aeration. However, they require significant space and aren’t always feasible in urban settings. Important note: Even “open” areas may have compacted soil from previous development, requiring restoration and tilling before planting.

3. Structural Soil

This medium uses stone to support pavement while providing soil for the roots in the gaps between the stone. While better than compacted soil, it offers only 20% usable soil volume, requiring five times more space than soil cells to achieve the same usable soil volume. It’s moderately priced and works well around utilities but often results in weak root systems.

4. Soil Cells

Currently the most effective solution for urban trees, soil cells provide high usable soil volume while supporting surface infrastructure. Though more expensive and time-consuming to install, they offer the highest success rate for tree establishment. Two main types exist:

• Post-and-base systems (85% usable soil volume)
• Open lattice structures like RootSpace (97% usable soil volume)

Hybrid Solutions

Combining different planting methods can maximize success while managing costs:

• Soil cells + open planting areas: Allows for smaller landscaped areas while maintaining adequate soil volume
• Soil cells + structural soil: Uses soil cells near the tree for structural root development and then structural soil beyond, where the fibrous roots are.

Tree Pit Layouts

Trees can be worked into cramped urban environments with the right planting medium and a creative layout. Several common configurations include:

• Back of curb, with tree grate, under pavement (typical for streetscapes)
• Square tree pits under pavement (common in plazas)
• Curved planters connected to soil cells or structural soil beneath paths (ideal for parks and larger landscapes)
  • connects open planters to each other so that roots can share soil space, without sacrificing walkway space
• Narrow open planting strips with soil cells or structural soil under nearby pavement (often used in streetscapes or retail fronts)
  • when planting strips have to be narrow to accommodate pedestrians but don’t provide sufficient soil volume, soil cells can be used to supplement planters beneath the pavement.
• Linked tree pits (can be used wherever trees are within 20 feet of each other)
  • This allows for 20% reduction in soil volume per tree as roots can share soil space
• Corner/offset designs (useful for corners and constrained spaces)
  • A tree will put its energy into growing roots where there is least resistance. So roots will be larger if they can only grow in one direction versus multiple directions. This is why the root ball can be set in the corner of a tree pit if needed, without sacrificing stability of the tree.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Working with Space Constraints

All planting methods can be adapted to work around utilities, though some require more careful planning than others. Some utilities, like pipes, can be worked into soil cell systems or structural soil. Others, like light poles, can be worked around and protected. Soil cells have proven successful even in historic cities with complex infrastructure networks.

The key to success lies in maximizing available space while ensuring adequate soil volume. When surface space is limited, high-efficiency solutions like soil cells become particularly valuable, allowing trees to thrive without compromising urban infrastructure.

View our RootSpace and Utilities Guide for design tips on working with or working around utilities with RootSpace.

Keep an eye out for Part 3 of our series, where we’ll dig deep into soil structure, long-term maintenance strategies, and other final considerations for achieving urban trees that thrive.