Components  
   
Intensely Urban Situations  
Trees in these areas are confined to sidewalk planting areas often between building facades and the street. Often times they are sharing space (usually underground) with utilities. The planting area is limited: sidewalks need to safely provide as much room as possible for pedestrians.  
   
1. Planting Options  

Figure 1. Example of tree plantings in an intensely urban setting. Note trench for tree interconnection, mulch and protective fencing.

There is no standard tree planting detail - the conditions must be assessed for every situation. The most important ingredient in urban tree planting is proper soil. Trees need as much soil as possible; an ideal urban situation provides continuous, interconnected improved soil that is protected from compaction by surrounded uses. For healthy crown growth, trees should be spaced approximately 30 - 40 feet apart. (Urban, J., no date; Figure 1).

Tree trenches provide the interconnected soil that street trees need for healthy root growth. Planting in linear trenches also allows trees to form continuous canopies that contribute to wildlife habitat and shade quality. Trenches to be approximately 6 ft. wide. (Urban, J.)
 
   
Disconnected tree pits for individual trees are not a good design alternative. They do not provide the continuous soil trees need. If only one tree is being planted, or there is no other option.  
   
2. Type of cover around trees  
  • Do not use tree grates around trees - they are expensive, require maintenance, and can damage trees if they are not sized and replaced as the tree grows (Urban, J., no date). Alternatives to tree grates are mulch, groundcover plants, and pavers. Research indicates street trees are subject to less damage if they are underplanted (w/ herbaceous plantings and shrubs) instead of surrounded by hardscape.
  • Mulch is best for all trees and will be addressed below under Tree Care. Unfortunately, intensely urban situations often are not suitable for mulch, but may be possible if combined with a low fence.
  • Groundcover plants provide great protection for street trees because they usually keep pedestrian traffic out of the tree's root bed. Again, a low fence adds additional protection. Further opportunities for these understory plantings are discussed in the "Broken Urban" section.
  • Flexible pavers can be used when there is no room to provide either of the above permeable soil surfaces. Pavers should be set in wide sand joints leaving at least 1 ft. of space at the trunk to allow for unimpeded tree growth (Urban, J., no date).
  
   
3. Soils  
Soil Volume: the graph below provides soil volume requirements for expected tree crown size at maturity.  
   

Figure 2. Tree damage in relation to root surface material

Structural Soil - Trees require loose permeable soils where their roots can grow and access rainwater or irrigation; however, these needs are in direct conflict with engineering specifications for compacted soil beneath roads and sidewalks. Recently, researchers at the Cornell University Urban Horticulture Institute have developed a new pavement substrate called "Structural Soil". This medium has high bearing strength for a stable pavement surface, but remains root penetrable and supportive of tree growth.
 
Structural soil is made of a precise combination of crushed stone, clay loam and a hydrogel stabilizing agent (Bassuk, Nina, 1997). (http://www.hort.cornell.edu/department/faculty/bassuk/uhi/pubs.html)  
   
4. Tree selection  
Trees will live longer, healthier lives by utilizing compatible site restrictions. For example, trees planted under utility lines may be topped. If street trees are planted under utility lines, it is best to choose a species that does not grow over 25' at maturity. Another alternative is to specify a plant grouping consisting of only understory shrubs. The City of Seattle Urban Forest Coalition provides extensive lists of appropriate street trees for a range of urban situations. The recommendations are divided into categories based on the width of the planting strip; for example, very narrow planting areas (less than 5 ft wide) require compact trees like the Washington hawthorn, Crataegus phaenopyrum. (City of Seattle Urban Forest Coalition, 1998; http://www.ci.seattle.wa.us/td/treeplant.asp).  
   
5. Tree care  
  • Mulching: All newly planted trees are covered with a 2"- 3" layer of mulch to protect the tree base, allowing soil to retain more water (City of Seattle Urban Forest Coalition, 1998). Mulch also discourages weed growth. Trees need to be weed and grass free for at least the first two growing seasons (Urban, J., no date). An 8 ft. diameter spread of mulch around a newly planted tree will contribute to improved root development (City of Seattle Urban Forest Coalition, 1998), but in more constricted planting situations, a minimum 6 ft. diameter is retained. Finally, always leave an air space between the base of the trunk and the mulch layer.
  • Irrigating - New trees need an established maintenance plan providing at least 5 gallons of water per inch of tree trunk diameter for at least the first dry season after planting (City of Seattle Urban Forest Coalition, 1998). Preferably, new trees should be watered for the first 3 dry seasons after they are planted. Irrigation should occur at a slow rate to allow water penetration into soil.
  • Staking - Staking is not recommended for new trees. Trees will not need staking if they are planted so that the side of the tree that faced north at the nursery continues to face north in its new planted location (Urban, J. no date). If trees are staked, then all stakes and staking equipment should be removed after the first growing season or they will interfere with the tree's growth.
 
www.ag.ohio-state.edu/~ohioline/hyg-fact/1000/1141.html  
www.na.fs.fed.us/spfo/pub/urbanforestry/techguide/planting.htm  
   
Broken urban situations  

Figure 3. A residential example of a broken urban setting

Less continuously paved than intensely urban situations, broken urban situations provide trees with increased opportunities for air, water, and nutrients. In these settings, street trees are planted between the sidewalk and the adjacent land use. A larger expanse of soil benefits trees and reduces conflicts between tree roots and sidewalks.
 
   
Ecosystem Function  

The root bed of urban trees is best protected by an understory or groundcover planting. In broken urban environments, there is an opportunity to go further with these protective plantings to achieve aesthetic and ecological benefits in addition to tree health benefits. Large trees with shrub and groundcover understory, especially native species adapted to the region (see Tree Selection above), offer species richness and structural complexity that can provide habitat for native animals and insects. Over time, these plantings may begin to form a matrix of patches and corridors, increasing opportunities for biodiversity within the urban environment. Findings from research into habitat potential of urban forest remnants for “red-listed” forest bird species indicates that a critical aspect of urban forest systems is the presence of corridors which extend the forest habitat type to link forested areas (Mortberg, 2000). Broken urban environments have the potential to serve this purpose on a micro-scale and are likely to show benefits when viewed cumulatively.

Another important ecosystem function of plant groupings is their ability to capture rainfall (in the leaves and through evapotranspiration), thereby reducing stormwater runoff in the urban environment. The structural complexity of plant groupings, especially over time, can mimic pre-disturbance land cover providing greater storage and uptake than alternative covers such as impervious surfaces and sod.