4.5.2 Design

There are numerous tree species appropriate for the eastern Washington with conditions ranging from sustained temperatures over 100 degrees in the summer to severe winter winds and cold weather. Successful planning where extreme growing conditions occur is essential. Absolute cold is not so much the problem as are drying winds that dehydrate plants growing in frozen soils. Wind protection, mulching, appropriate soil structure and careful late-autumn watering helps facilitate healthy tree growth in areas of extreme weather.

While cold is not an insurmountable challenge, snow storage in the immediate vicinity of trees could create problems if the selected trees were not adapted to heavy snow conditions. Whether along roadways, urban streets, or parking areas, appropriate site planning for adequate snow storage can play an important role in maintaining the health of trees and planting areas.

Preparation for tree planting in eastern Washington is particularly important as most tree nursery stock is obtained from nurseries located in the mild climates of western Oregon and Washington. Preparation of tree planting holes and the specification of appropriate planting topsoil are of the utmost importance. Urban (2008) recommends a minimum depth for planting soil of 30 to 48 inches. See additional soil depth and volume discussion below.

The remainder of this section is divided into seven parts:
  • Site Assessment and planning.
  • Planting size.
  • Spacing.
  • Drainage.
  • Soil depth and volume.
  • Soil amendments.
  • Increasing soil and rooting volume.

4.5.2.1 Site Assessment and planning
Planting and retaining healthy trees requires space and investment. Realizing the substantial benefits of mature trees requires engaging the designer from planning through construction phases, whether new construction or a retrofit. Site assessment to inform soil strategies and species selection is important for healthy tree growth and to reduce potential problems with competing uses. The initial site assessment for location and type of tree should include:
  • Available above-ground growing space.
  • Below ground root space and ground level planting area relative to pavement, buildings, and utilities.
  • Type of soil and availability of water.
  • Overhead obstructions.
  • Vehicle and pedestrian sight lines.
  • Proximity to paved areas and underground structures.
  • Proximity to property lines, buildings, and other vegetation.
  • Prevailing wind direction and sun exposure.
  • Maintenance.
  • Slope and topographic features.
  • Proximity to snow storage areas.
  • Local tree lists of required and prohibited trees.

Additional environmental, economic, and aesthetic functions, such as shade (reduced heat island effect), windbreak, privacy screening, air quality, and increased property value should also be considered when determining the use, type, and placement of trees.

Many of the key decisions for designing with trees in eastern Washington depend on existing soil conditions. Soil analysis for trees should include: understanding historic uses, extent and result of disturbances, soil texture, compaction, permeability, barriers and interfaces in the soil profile, and chemical characteristics (Urban, 2008). Urban soils are often degraded from construction activities. If the existing soil or structural soils are used as the planting material, particular attention should be given to soil pH, which is often high due to concrete/construction debris and can cause nutrient deficiency and other problems. The ideal soil pH for most trees is 5 to 6.5 (Day and Dickinson, 2008).

Once the basic site assessment and soil analysis is compiled (see Chapter 2: Planning for LID), the following guidelines can be applied for site layout to incorporate trees (Urban, 2008):
  • Plant in appropriate areas characterized by quality soils and adequate soil volume for the appropriate tree species.
  • When designing spaces for trees it is important to reduce impervious surfaces around or near the tree planting areas. Planting beds should be of an adequate size for the tree selected and special attention should be given to increasing soil and rooting volume. Tree planting areas should ideally be at least 8 feet in width.
  • Do not pave near a mature tree’s trunk flare. The trunk flare is the transition area between the base of the trunk and root crown and is often 2 to 3 times the trunk diameter (trunk diameter measured at 4 feet above ground).
  • Use permeable pavement for hard surfaces near trees to allow gas exchange and to promote soil moisture.
  • Protect the tree and tree pit soil from surrounding uses (e.g., pedestrians, vehicles, ongoing maintenance activities).
  • Avoid planting trees in areas where heavy snow storage occurs.
  • Select trees that minimize conflicts with existing or planned utilities.

4.5.2.2 Planting size
A 3- to 4-inch caliper tree is the optimum size for planting deciduous trees in the urban setting (Urban, 2008). For coniferous trees, a planting height of 6 to 8 feet translates to a 3- to 4-inch trunk caliper. Plant availability for trees of a larger size can be a challenge. When a large quantity of trees of the same species is specified, it is important to select a size that can be readily provided by suppliers.

The time to recover from transplant shock is approximately 6 to 12 months per caliper inch depending on latitude (Urban, 2008). Planting larger trees is appealing to provide a more mature appearance initially; however, transplant shock may last longer and maintenance during recovery may be more extensive. In contrast, 3- to 4-inch caliper trees will likely recover faster, with growth eventually surpassing the larger tree with less initial care (Urban, 2008).

4.5.2.3 Spacing
Appropriate spacing of trees is dependent on the species selected and the planting environment. For example, a London Plane tree (Platanus acerifolia) should be planted no closer than 40 feet on center and preferably 50 feet on center because of its large mature size. Smaller flowering trees can be placed much closer, perhaps 25 to 30 feet on center. In some settings, designers and arborists chose a closer tree spacing to achieve a more mature initial planting design effect, with the intent of removing trees at a future date as they reach more mature sizes. Tree spacing should be carefully considered based on local conditions and with the consultation of a landscape architect or arborist.

For parking lot tree planting, landscape codes often specify minimum tree spacing requirements. Some codes provide a performance standard approach to both tree spacing and size of required tree by requiring a certain percentage of paved area be covered in shade within a specified time frame (e.g., 50 percent of the paved area shaded within 5 years).

Generally trees should be planted to allow mature tree crown development. Ideally, tree planting beds should be 8-feet-wide. Where significant snow storage is anticipated, trees should be protected and planted away from significant snow accumulations. Plowed snow is denser and can be heavily laden with deicing chemicals and salts – either of which can be detrimental to healthy plant and tree growth.

Trees should be setback an adequate distance from plow lanes (six feet minimum) in order to be protected from plow blade damage. In parking areas where snow accumulation can be significant, parking and drive lanes should be arranged to allow ease of snow plowing and to facilitate the protection of planting areas. Larger planting islands for trees should be located and aligned at the ends of parking rows. Intermediate tree planting islands can be provided at regular intervals in larger parking lots to visually break-up expansive asphalt areas.

4.5.2.4 Drainage

Assessment of subgrade soils, groundwater levels, and site drainage patterns should be used to determine soil water and optimum tree planting conditions. In general, the tree planting pit or reservoir in the tree rooting zone (18 to 24 inches) and above under-drain (if installed) should drain down within 48 hours to encourage aerobic conditions and good root distribution through planting pit for many tree species (Bartens et al., 2009).

However, there are species more tolerant of prolonged saturated conditions. If the site assessment determines there is potential for extended ponding or dense, compacted soils are present, consult an engineer for appropriate drainage strategies and a landscape architect or arborist for appropriate tree species.

With adequate subgrade infiltration rates, tree planting areas can provide on-site retention of stormwater runoff. Careful assessment of subgrade soils, groundwater levels, and site drainage patterns should be performed to determine soil water and optimum tree planting conditions (Urban, 2008).

Increasing the volume of soil and preventing compaction of existing soil in the tree planting areas for roots also increases the volume for stormwater storage and treatment. See the Section below on soil depth and volume.

4.5.2.5 Soil depth and volume

Urban (2008) recommends a minimum depth for planting soil of 30 to 48 inches. This depth should extend for a 10-foot radius around tree in lawn areas.

Recommendations for adequate soil volume vary significantly for trees planted in conventional soil. Lindsey and Bussuk (1992) recommend approximately 8 cubic feet per 10 square feet of crown projection for a typical silt loam soil to provide the volume necessary to support adequate root structure. Urban (2008) recommends determining required soil volume depending on soil type, water availability and tree size (crown projection or trunk diameter), at a rate of 1 to 3 cubic feet of soil per square foot of tree crown area. Where irrigation is provided, 1 cubic-foot of soil for every square-foot of crown area is recommended. For trees without irrigation, soil volume should be increased to 3 cubic feet for every square-foot of crown area.

Several strategies are presented below for increasing the soil depth and volume to promote healthy trees.

4.5.2.6 Soil amendment for trees
If possible, stockpile and reuse existing soils for tree planting. Relatively fine- grained soils can be reused and support healthy tree growth. For adequate drainage and tree health, Urban (2008) recommends avoiding topsoil that has more than 35 percent clay, 45 percent silt, or 25 percent fine sand. Loam, sandy loam, and sandy clay loam provide good textural classifications for supporting healthy tree growth (Urban, 2008).

If stormwater is directed to the tree planting area, a designed soil mix may be necessary to achieve adequate infiltration and drain-down characteristics. The water holding, organic matter, and chemical characteristics of the soil must be compatible with the water needs and other cultural requirements of the tree.

A variety of materials are available to amend existing soils or design a specific soil mix. Mineral soil amendments alter soil texture and improve infiltration and water holding characteristics. Common materials used in tree planters and planting areas include: sand, expanded shale, clay and slate, and diatomaceous earth (see Urban, 2008 for detailed descriptions for using mineral amendments).

Native soils across eastern Washington are relatively low in organic matter. Biologic and organic amendments should be used to improve organic matter content, infiltration capability, nutrient availability, soil biota, and cation exchange capacity, as appropriate. Biologic amendments include mycorrhizal fungi spores, kelp extracts, humic acids, organic fertilizers, and compost tea. If tree planting soil is poor quality, biologic amendments generally only offer a temporary improvement for tree growth.

4.5.2.7 Increasing soil and rooting volume
There are four primary strategies to improve the subsurface environment for trees and provide stormwater infiltration in urban settings:
  • Rigid, load-bearing cells that are filled with uncompacted soil.
  • Structural soils.
  • Creating root paths.
  • Connecting to adjacent soil volume (Urban, 2008).

Soil Cell Systems

Soil cell systems are modular frames (base and pillar) with a deck that supports the pavement above and creates large spaces for uncompacted soil and tree roots. DeepRoot Green Infrastructure developed the Silva Cell, which is a common type of rigid load-bearing soil cell for trees. The decks are often designed for AASHTO H-20 loading (see Figure 4.5.3). Many utilities can be installed within and through the cells; however, utilities require planning and careful consideration. Many types of soil can be used to fill the cells for a rooting media, including imported soils designed for the specific tree or excavated soils (including heavier dense soils with higher clay content) amended with compost if necessary (ASLA, 2010). An advantage with soil cells is that more than 90 percent of the volume created by the cell is available for soil.

When soil cells are filled with a soil mix that meets Ecology’s treatment requirements, such as bioretention soil mix (See Section 4.4.2.2: Bioretention components), the system may be designed to be functionally equivalently to a bioretention facility. See Ecology’s Equivalent Technology website (www.ecy.wa.gov/programs/wq/stormwater/newtech/equivalent.html). Soil cells designed to be equivalent to bioretention can greatly increase the return on investment for large trees by reducing or eliminating the need for downstream BMPs to meet stormwater Core Elements.

Structural Soils
Structural soils provide a porous growth media and structural support for sidewalks and street edges. Cornell University (CU Structural Soil™) developed one of the first structural soils in the early 1990s and others have since developed load-bearing growth media (e.g., Stalite). Structural soils are a mix of mineral soil (typically a loam or clay loam with at least 20 percent clay for adequate water and nutrient holding capacity) and coarse aggregate (typically uniformly graded ¾-inch to 1½-inch angular crushed stone) that, after compaction, maintains porosity (typically 25 to 30 percent) and infiltration capacity (typically > 20 in./hr.). Current research and installation experience suggests the following when designing with structural soil:
  • Structural soil can be used under all or part of the paved surfaces adjacent to trees to provide the necessary soil volume. Where structural soil is placed adjacent to open graded base aggregate, geotextile should be used to prevent migration of the fine aggregate in the structural soil to the more open graded material (Bassuk, 2005).
  • Soil depth: 24 inches (minimum) to 36 inches (recommended) (Bassuk, Grabosky, and Towbridge, 2005).
  • Compaction: 95 percent proctor (Bassuk, Grabosky, and Towbridge, 2005).
  • Tree pit opening: If the tree pit opening is at least 5 feet x 5 feet, a well-drained top soil can be used in the planting area. If the opening is smaller, structural soil can be used immediately under and up to the root ball (Bassuk, Grabosky, and Towbridge, 2005).
  • Available soil: the structural aggregate uses approximately 80 percent of the available space; therefore, approximately 20 percent of the total planting volume is available soil to support tree growth.
  • Soil volume: 2 cubic feet for each 1 square-foot of crown projection (mature tree) is a well accepted industry standard. Because the structural aggregate uses approximately 80 percent of the available space, 10 cubic feet of structural soil for each 1 square-foot of crown projection (mature tree) may be needed.
  • Planters with impervious walls: openings filled with uncompacted soil can be used to allow roots to access surrounding structural soil (Bassuk, Grabosky, and Towbridge, 2005).
  • Tree species: Use species that are tolerant of well-drained soil and periodic flooding.
  • Drain down: Structural soil reservoir should drain down within 48 hours to encourage good root distribution through planting pit (Bartens et al., 2009).

Many structural soils are proprietary mixes distributed through licensed providers. Sand-based structural soil (SBSS) is an urban tree planting system that is not proprietary. SBSS consists of a uniform gradation of medium to coarse sand (typically 30 inches deep) mixed with compost (2 to 3 percent by volume) and loam to achieve approximately 8 to 10 percent silt by volume.

In general, the saturated hydraulic conductivity should be approximately 4 to 6 inches per hour. The uniformly graded sand maintains porosity and infiltration capacity when compacted; however, the load-bearing capacity of the mix is reduced due to the uniform particle size. Accordingly, crushed stone is used between the sand and surface wearing course (see Figure 4.5.4).

If using structural soil to try to meet stormwater treatment requirements, the designer will need to demonstrate that the soil mix meets Ecology’s Soil Suitability Criteria (Section 5.4.3 of the 2004 SWMMEW).

A subsurface irrigation port that can be accessed from the surface of the tree pit or drip irrigation should be incorporated for initial establishment of trees and subsequent irrigation if necessary (ASLA, 2010). As with all urban tree systems, excess water and anaerobic soil conditions can impair or kill trees and subsurface drainage layers or under-drains should be considered to manage soil moisture on subgrades with low permeability. Structural soils can be used in conjunction with permeable pavement (Haffner, 2007).

Contact authorized distributors and see Day and Dickinson (2008) for guidelines on specific structural soil products.

Creating Root Paths
Root paths are a technique to connect planting areas, interconnect tree roots, or guide roots out of confined areas to soil under pavement or adjacent to paved area that has the capability to support root growth (e.g., uncompacted, adequately drained loams). The actual root paths add only small amounts of rooting volume. The path trenches are typically 4 inches wide by 12 inches deep and filled with a strip drain board and topsoil. Root paths are excavated with a standard trenching machine, placed approximately 4 feet on center, and compacted with a vibrating plate compactor to retain subgrade structural integrity for pavement. The trenches should be extended into the tree planting pit a minimum of one foot and preferably within a few inches of the tree root ball (Urban, 2008).

Connecting to Adjacent Soil Volume
Soil trenches are used to increase soil and root volume, connect to other tree planting areas, and importantly, connect to larger areas with soil that have the capability to support root growth (e.g., uncompacted, adequately drained loams). The trenches are typically 5 feet wide with sloped sides for structural integrity and filled with topsoil or a designed soil mix. The installed soil is lightly compacted (e.g., 80 percent Proctor) with a gravel base placed on top of the soil to increase support for the sidewalk. The sidewalk is reinforced with rebar and thickened to span the soil trench. The thickened portion should extend a minimum 18 inches onto the adjacent compacted subgrade. An under-drain may be necessary depending on subgrade soil with low infiltration rates and if stormwater is directed to the tree planting area (see Section 4.5.2.4: Drainage, and consult with the project engineer for drainage requirements). Provide subsurface irrigation conduit preferably from stormwater or harvested water in areas with less than 30 inches of annual precipitation (Urban, 2008).


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Figure 4.5.3
Silva cell filled with bioretention soil mix and topped with permeable pavers. Source: Otak.


Fig4-5-4_SoilStructure_Section
Figure 4.5.4
Sand-based Structural Soil Section. Source: AHBL, Inc.