Design Criteria
  Structural Tree Soil
  Amended Native Soil
Plans & Details
Monitoring & Lessons Learned
Cost Considerations
Additional Resources
Site Specifications

Specifications were developed for each site. Download them here:

Additional Info
Each material has been incorporated for a specific reason. Read on to learn more and download additional information and others' specifications.
Structural Tree Soil

Purpose: A material that supports traffic loads & the long-term tree health by providing structure with the angular rocks and nutrition in the voids between them.

What is it?: We used a proprietary blend, which started with an open-graded (i.e. all the same or very similar diameter) rock with no fines, similar to the kind of rock that might be used under a porous pavement, with 40% void ratio. Instead of preserving those voids, though, our mix incorporated some clay, biochar, mychorrhyzae (mushroom roots), biota, GeoHumus, and a stabilizer. This material filled some of the voids, but still left us with a material that had about 25% void ratio. Mycorrhyzae can greatly improve the trees' access to water and nutrition and are highly recommended.

Placement: On either side of the outsides of the trench where cars will park. After placement, it will be paved.

Installation tips: Whether you use a proprietary blend is up to you and your contractor. The City of Olympia mixed their own with good results for the trees, but the MOST IMPORTANT THING is that you don't under- or overcompact this material. We used a lightweight vibratory compactor and tested each lift every 25 feet or so along the two outside trenches to ensure that the material was compacted 85-90% compaction. (This may ellicit eye-rolling from the technician, as it did on one of our sites, since this spacing greatly exceeds typical spacing for road subbase testing. Stand your ground or get professional assistance on spacing from your geotechnical engineer and then stand your ground.)

Moisture content really affected the ability for us to compacct, but in the end, once you've got your compaction and desired density, moisture isn't really that important.

How to determine the appropriate depth of material: Our geotechnical engineer had the material tested to determine the density at 85 to 90% compaction, and he inspected the site soils to determine their structural stability under a wet, uncompacted condition. He then determined the depth of structural soil, at 85% compaction, that would be needed to support the predicted traffic loads.

Click here to download/view: .

Parking Forest related specs and info for each site:
Info from other projects/approaches:
Amended Native Soil

Purpose: To support the short-term health of the tree during establishment.

What is it? Just what it sounds like. We took our native clay soil (that infiltrated at a rate of about 1 inch/hour) and mixed in compost and Permamatrix, which includes mycorrhyzae, biota, and biochar. (See plans and specs for quantities.)

Placement: In the middle trench where cars may overhang but not drive on. After placement, it will be finished in crushed rock.

Installation tips: Water or boot compact the soil in 8-12" lifts, so it doesn't settle and leave you with a tripping hazard at the end of the project.

How to determine the appropriate depth of material: Convention with bioretention facilities (ex. rain gardens, stormwater planters) often assumes that slower infiltrating soils should be removed and replaced with faster draining soils. Since tree roots often grow to a depth of 3 feet, then a designer might remove and replace 3 feet of soil.

We disagree with convention. Native trees are adapted to native soil.The best thing for those trees is the clay that's there, as long as it doesn't have standing water all the time, so for ease of construction, we specified the same depth of amended native soil as what you might find for the structural soil.


Purpose: To restore the long-term permeability of the clay soils that had to be removed from our trench during construction (i.e. to quickly restore structure) and to feed the trees.

What is it? The US Composting Council defines compost as: "a carefully decomposed, sanitized, and stabilized organic soil amendment that encourages plant growth." The US Composting Council has a Seal of Testing Assured (STA) program that ensures that compost suppliers are testing their compost for various important physical properties as well as some harmful pollutants and that they make those tests available upon request. The seal doesn't assure that the compost is ideal for your use, just that it's been tested. Nonetheless, this transparent program is more likely to deliver a high quality product that's stable and good for your trees.

Placement: This material was folded into our amended native soil using a bucket on the backhoe. You can also do this by hand with a garden rake (best) or shovel.

How to determine the appropriate quantity: The Washington State University Extension Service in Puyallup has found that folding compost into clays compacted and disturbed during construction can restore and enhance the long-term permeability, much more so than just ripping. They have recommendations for lawn and garden area quantities at their outreach website for builders and contractors.

Click here to visit:

Crushed Rock

Purpose: To provide a convenient walking surface when stepping through the 4' wide native soil planting trench.

Placement: This material was placed on top of the native amended soil mix at a depth of 3 to 4 inches.

What is it? Crushed rock, in this case, refers to an angular rock that is open-graded. Open-graded means that the rocks are all about the same size. Smaller is better than larger, both for aesthetics and for foot traffic, so we used a specification known as 1/4"x10. Both numbers in the designation refer to a seive size that the material will pass through and they are close together in diameter.

How to determine the appropriate quantity: We took a guess, hoping that 3 to 4 inches of rock would spread foot traffic loading out enough that the amended native soil below would not be overly compacted. We also didn't want to leave the tops of the trees with so little dirt that they wouldn't do well.


Purpose: In addition to the numerous benefits of trees in development projects, as described on the Benefits page, vegetation in stormwater facilities is needed to protect and improve the long-term permeability of the soil and provide water quality treatment. The more roots, the more benefits, so trees in stormwater facilities are especially beneficial.

In general, vegetation works to take up pollutants. Some will get broken down. Some will simply get stored in the cell walls throughout the structure of the plants. In addition to the work that plants do on their own, the roots provide habitat for a variety of microorganisms that concentrate on their roots. Plant roots are like the "dinner table" of soil, pulling in food -- pollutants -- and water. These microbes multiply like crazy on the plant roots and are very effiicient at breaking down hydrocarbons such as oils. Microorganisms attract larger soil animals like worms and beetles to the "dinner table" underneat the surface of the facility. These larger soil animals need air and water, and constantly move through the soil, creating structure that provides the long-term permeability of the facility.

What is it? I think you know what a tree is, but we used the following specifications:

  • 1.5-inch caliper deciduous trees
  • 5 to 6-foot high evergreen trees (unless they're deciduous evergreens, like Madrones)

Tree should:

  • Be able to tolerate your seasonal variation. In our case, we need trees that can survive our regular 3-month summer drought and high seasonal groundwater table.
  • Be able to tolerate temporary inundation. Our facilities are designed to drain out in 30 hours because of our rainfall distribution (Type IA). If you live in an area with a different rainfall distribution (Type I, Type II, or Type III), your drain out time is likely to be longer. Choose a tree that can withstand whatever indundation period the facility is designed for or, conversely, design the facility to drain out in a period that the tree is adapted to.
  • Not drop materials that could cause someone to slip and fall, such as slippery or heavy fruits or nuts.
  • Not be brittle. Fast growing trees are often short lived and are brittle.

Other considerations you might have in choosing tree species are:

  • Will your trees be exposed to saltwater within the 3-foot rooting depth?
  • Are you in a riparian area? Natives only should be used in this case, since the areas alongs streams are particularly sensitive lands.

Some Planting Tips: Visit the International Society of Arboriculture for guidance on how to plant a tree. Common mistakes are planting it too high or too low, which can significantly impact tree health.

Location: Unless you have parking spaces longer than 18 feet, you won't have enough space to place the trees behind some kind of barrier. You'll need the space between the parking spaces for car overhang area, so plant trees in line with the parking striping. See the plans and details of each site to see where we located our trees for head-in angled and straight-in parking.

Lessons Learned: You may want to add a sign that says "No Backing Into Space Allowed" or something to that effect. The overhang for the back of a truck is much longer than the space needed to overhang the front of a truck.

A Word on Trees in Porous Pavement Parking Lots: At the THPRD redevelopment project, there was a lot of concern initially about leaves clogging the porous pavement. Typical maintenance practices used with porous pavement like vacuuming and leaf blowing are effective at protecting it even when trees are nearby.

Wheel Stops

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Purpose: Wheel stops were used to keep cars from driving into the exposed 4-foot wide amended native soil strip so that long-term permeability can be protected and also so that no one can drive into the tree trunk. We avoided 16- or 18-inch deep poured curbs because those curbs have 6 inches of curb above ground and 10" to 12 inches of concrete below. Since most of the feeder roots of a tree grow in the first 12 inches of depth, we didn't want to block either the structure or feeder roots from growing over into the structural soil.

What is it? This is a 6-foot long, 6-inch tall piece of site furniture secured to the pavement, usually with two steel rods. Acceptable materials include concrete, plastic, and recycled plastic. Wheel stops with feet, instead of ones like at the PCC site where the entire bottom is in contact with the ground are preferable. Wheel stops with feet do a better job at preventing erosion, since runoff isn't concentrated along the entire 6-foot length.

Placement:At the very edge of the pavement, centered on the parking space or on the parking space striping..

How to determine the appropriate quantity. Usually, each parking space gets its own wheel stop; however, centering the wheel stop on the parking space striping will stop the right wheel of one car and the left wheel of the car in the adjacent parking space.

Creative Commons LicenseParking Forest Website by Green Girl LDS LLC is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. We believe the term "Parking Forest" was coined originally by Brian Wegener of the Tualatin Riverkeepers in an email on June 29, 2013.
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