Green streets

Growing conditions

Image credit: Stichting Open Boek

Trees need quality soil to grow

Soil should provide good air circulation, organic content, porosity, water storage capacity and soil biology. Soil biology should be sufficient to generate active decomposition in the ground to convert some of the compounds from the dust particles collected by the trees into harmless compounds. Remove sterile building sand and replace with the appropriate soil mixture, depending on the situation.

Roots need room to grow

Prevent the ‘flower pot effect’ by providing sufficient underground growing space with suitable conditions. The size of the tree planting area depends on the surrounding uses above ground (park or yard vs. street or building), the size of the tree, and how high the water table is. At the design stage, the mature size of the tree should be considered. Excessive pruning to accommodate adjacent buildings, powerlines, etc. denies the tree its essential energy (leaves are the lungs of the tree).

Trees have specific water needs

Maintenance and water should be budgeted into the project to guarantee optimum growth. This is especially important in the first year after planting. Avoid digging planting beds deeper than 20cm above the groundwater table. Caution is advised in using the planting beds around trees as drainage for surrounding pavement because of the damage that may result from  excess water or salt.

Do not allow soil compaction to occur

If too much compaction occurs under pavement (traffic areas) then root growth is limited due to less porosity, air circulation and water drainage / availability. Compacted soil restricts water infiltration into the root zone and groundwater table.

Tree roots need adequate oxygen

This can be achieved by the proper soil type with porosity or with a perforated pipe network with 40% perforation under the pavement. Pavement which is damaged by roots is usually caused by roots searching for oxygen closer to the surface


Create the proper growing conditions for urban trees so they are healthy and can perform optimally.

 

Characteristics of soil used for trees in urban settings

tree soiltree sandtree granulate
organic matterhigh content (5-7%)4-5%
structureopen and loosemix of coarse sand and organic matter60% rocky material mixed with 40% compost, peat or clay
porosity65%50%
amount of trafficno traffic within canopy radiuslight trafficmedium to heavy traffic
locationopen ground: parks, green areas, green strips between paving and grass/plantingunder sidewalks, bicycle paths, parkingunder streets (with good circulation)
min. pointsnot suitable under pavement and trafficmust be professionally mixed to avoid compaction, lack of oxygen and drying outmust be professionally mixed because it does not mix evenly, also difficult to dig holes for utility work

Benefits of good conditions

  • Trees are able to reach maturity
  • Trees are healthier and more vigorous
  • Trees perform optimally (provide shade, clean the air, etc.)
  • Trees require less maintenance and control which saves money in the long term
  • Less problems such as damaged paving, diseases and pests
  • Healthy roots have a positive effect on the capacity of the ground to buffer water

“Green in the city … provide the right conditions … and nature will do the rest.”

De bomenplanner

 

New and existing development

In new development:

Remove building sand in all tree and plant beds and replace with planting soil. Design not only above ground but also underground so there is ample room for trees to grow.

In existing development:

Street renovation should not only be based on traffic circulation but also the optimal growing space for the new or existing street trees. Street profiles, space underground and soil type should be adjusted to prevent problems such as paving damage due to roots.