Classics
The following article has been selected because it is deemed very popular or very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
The Stockholm Solution - Case Study
Edited by Len Phillips
Stockholm, the capital city of Sweden has developed a very successful tree installation program that many experts believe produces better tree growth than almost all the other methods (see Topic 18) developed for installing trees in downtown city sidewalks. Here is the Stockholm Solution story.
Background
In 2001, Bjorn Embren was appointed to review the tree installation program in Stockholm. He began a working group including Britt-Marie Alvem, Örjan Stål, and Alf Orvesten, all well-known experts and researchers of tree roots. They also consulted with the Agricultural University of Stockholm and the University of Helsinki. They conducted a review of how trees grew in the urban environment and determined that soil conditions are most important for the establishment, development, and survival of trees in the city. They also collaborated with Klaus Schroder, a retired tree officer from Germany, who is credited with being the first person in Europe to work with structural soil.
In Stockholm center, about 20,000 street trees were dying because the city soils were lacking oxygen, water, and organic matter, plus a surplus of road salt, carbon dioxide, vehicle compaction, utility installation, and maintenance disruption were poisoning the trees.
Construction practices before 2001 required that all pavement be heavily compacted so rain water would run to the storm drains instead of the soil to discourage tree roots from growing under the pavements and causing pavement failure. Over time, the lack of water in the soils caused a decline in the trees, but more importantly, older buildings began to settle, shift, and destabilize, often requiring complete reconstruction.
In his search for answers, Mr. Embren and his team, discovered that along the railways in Stockholm, trees were growing extremely well in the stone ballast beside the tracks. The rocks in the ballast were large but stable and perhaps the same large stone could be used under the pavements in the city to allow root growth in the voids between the stones, and the stone would transfer the vehicle weight to the solid soil at the bottom of the installation pit. He also determined that the best soil conditions for tree roots surrounding the stone should consist of a homogeneous soil where the particle size distribution and humus content is mixed throughout the upper 12 inches (30 cm) of soil and the same soil minus the humus is located under this “topsoil”.
Based upon all the experts' research, and a process of trial and error, the tree planting team in the city developed a structural soil that consists of layers of 4 – 6 inch (10-15 cm) crushed rocks, installed in 10 – 12 inch (25-30 cm) thick layers. This resulted in about 25% – 30% pore space that is filled with soil. The rocks could be any available native stone, but Mr. Embren felt it would be much better to use broken up recycled concrete sidewalks instead of the native granite. Thus began an effort to install new trees in stone filled trenches throughout the city.
Planter Construction
A deep trench or pit is dug 30 – 40 in. (0.8-1 m.) deep and rocks are placed at the bottom 24 inches (60 cm). When the first layer of stone is compacted, soil is lightly sprinkled on top of the rocks. The soil is then washed into the voids between each stone with high pressure water. The process is repeated until the voids in the rock are filled with soil. A slow-release fertilizer is sprinkled over the stone/soil mix of each layer. A second layer of stone is added on top of the previous layer and soil with humus is washed in to this layer of stone.
On the top of the compacted stone, corner pads are placed for the installation of a square precast concrete box for the tree. The bottom and holes on the side of the box are open to the compacted stone/soil mix. When the project is completed, the trees are planted in the box so the roots may grow down and into the stone/soil planting area. If the tree needs replacement, the old tree is easily removed from the concrete box and a new tree installed in the same box.
When the structural soil has reached the finished grade, a layer of finely crushed stone is laid on the top to level off the top of the stone. Geotextile fabric is laid over the entire installation pit followed by a base material for the pavement and the surface layer for the specific paving of the road, sidewalk, or bike path to be installed. The fabric prevents the pavement base material from migrating down into the planting soil mix and prevent roots from growing up into the base material.
The nutrient rich homogeneous mix of soil guarantees a good rooting space while the rock meets the bearing capacity demands for heavy traffic. The planting loam must also contain 5% – 8% humus, 4% – 8% clay, and a fertilizer based on need determined by a soil test. Mr. Embren developed structural soil combinations for the four different soil types found in the city. Each tree needs a minimum of 500 cu. ft. (15 cu. m.) and a space at least 7 ft. (2 m.) wide for small trees and 15 ft. (4 m.) wide for large trees. The length should be as long as possible to achieve a mature, healthy tree. Whenever possible the tree pits are connected together so the root space can be shared by all the trees.
Water Treatment
The whole area is also used for stormwater containment. Drainage basins sit in the top of stone/soil mix so water running off the surface pavement is able to flow down, through the stone and into the root zone of the trees. As the water flows through the stone/soil mix it is used by the trees. Any surplus water is emptied into the original storm drain for the street. The catch basins in the new construction are built with a metal screen near the bottom where oxygen can also enter the soil/stone mix. The oxygen is absorbed by the tree roots and prevents stagnation of the water. Catch basins beside the street curbs and building gutters also provide water to the tree. Fortunately, salt used on the streets in winter does not seem to cause a problem in the soil or with the tree roots because of the high volume of water flowing through the stone/soil mix.
Dealing with Trees
New trees of the right size and species to fit the planned dimensions and location should be delivered to the site and installed in the conventional manner within the concrete box. New trees are generally large, balled and burlapped, and installed directly on top of the stone base and in contact with the soil filled pores. If the existing trees are to be retained, the soil should be washed off the roots of the tree. The roots are then covered with a small size stone near the trunk, that gradually increase in size to meet the large stone installed in the trench. Soil is washed into this stone as it was in the rest of the installation trench.
Like every urban tree installation, several factors must be considered before actually installing the tree.
Since the Stockholm Solution began, over 2,000 projects have been completed within the city, and the Swedish national government has accepted this concept for all future projects in the country. This program resulted in tree growth that is equal to or better than trees growing in nearby parks. It seems that all trees, regardless of species have thrived in this program. Tree experts from nurseries and other cities all remark at how well the trees in Stockholm look. Their color is dark green and leaf count is much higher than expected for trees growing in a typical urban environment.
Additional information:
The following article has been selected because it is deemed very popular or very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
The Stockholm Solution - Case Study
Edited by Len Phillips
Stockholm, the capital city of Sweden has developed a very successful tree installation program that many experts believe produces better tree growth than almost all the other methods (see Topic 18) developed for installing trees in downtown city sidewalks. Here is the Stockholm Solution story.
Background
In 2001, Bjorn Embren was appointed to review the tree installation program in Stockholm. He began a working group including Britt-Marie Alvem, Örjan Stål, and Alf Orvesten, all well-known experts and researchers of tree roots. They also consulted with the Agricultural University of Stockholm and the University of Helsinki. They conducted a review of how trees grew in the urban environment and determined that soil conditions are most important for the establishment, development, and survival of trees in the city. They also collaborated with Klaus Schroder, a retired tree officer from Germany, who is credited with being the first person in Europe to work with structural soil.
In Stockholm center, about 20,000 street trees were dying because the city soils were lacking oxygen, water, and organic matter, plus a surplus of road salt, carbon dioxide, vehicle compaction, utility installation, and maintenance disruption were poisoning the trees.
Construction practices before 2001 required that all pavement be heavily compacted so rain water would run to the storm drains instead of the soil to discourage tree roots from growing under the pavements and causing pavement failure. Over time, the lack of water in the soils caused a decline in the trees, but more importantly, older buildings began to settle, shift, and destabilize, often requiring complete reconstruction.
In his search for answers, Mr. Embren and his team, discovered that along the railways in Stockholm, trees were growing extremely well in the stone ballast beside the tracks. The rocks in the ballast were large but stable and perhaps the same large stone could be used under the pavements in the city to allow root growth in the voids between the stones, and the stone would transfer the vehicle weight to the solid soil at the bottom of the installation pit. He also determined that the best soil conditions for tree roots surrounding the stone should consist of a homogeneous soil where the particle size distribution and humus content is mixed throughout the upper 12 inches (30 cm) of soil and the same soil minus the humus is located under this “topsoil”.
Based upon all the experts' research, and a process of trial and error, the tree planting team in the city developed a structural soil that consists of layers of 4 – 6 inch (10-15 cm) crushed rocks, installed in 10 – 12 inch (25-30 cm) thick layers. This resulted in about 25% – 30% pore space that is filled with soil. The rocks could be any available native stone, but Mr. Embren felt it would be much better to use broken up recycled concrete sidewalks instead of the native granite. Thus began an effort to install new trees in stone filled trenches throughout the city.
Planter Construction
A deep trench or pit is dug 30 – 40 in. (0.8-1 m.) deep and rocks are placed at the bottom 24 inches (60 cm). When the first layer of stone is compacted, soil is lightly sprinkled on top of the rocks. The soil is then washed into the voids between each stone with high pressure water. The process is repeated until the voids in the rock are filled with soil. A slow-release fertilizer is sprinkled over the stone/soil mix of each layer. A second layer of stone is added on top of the previous layer and soil with humus is washed in to this layer of stone.
On the top of the compacted stone, corner pads are placed for the installation of a square precast concrete box for the tree. The bottom and holes on the side of the box are open to the compacted stone/soil mix. When the project is completed, the trees are planted in the box so the roots may grow down and into the stone/soil planting area. If the tree needs replacement, the old tree is easily removed from the concrete box and a new tree installed in the same box.
When the structural soil has reached the finished grade, a layer of finely crushed stone is laid on the top to level off the top of the stone. Geotextile fabric is laid over the entire installation pit followed by a base material for the pavement and the surface layer for the specific paving of the road, sidewalk, or bike path to be installed. The fabric prevents the pavement base material from migrating down into the planting soil mix and prevent roots from growing up into the base material.
The nutrient rich homogeneous mix of soil guarantees a good rooting space while the rock meets the bearing capacity demands for heavy traffic. The planting loam must also contain 5% – 8% humus, 4% – 8% clay, and a fertilizer based on need determined by a soil test. Mr. Embren developed structural soil combinations for the four different soil types found in the city. Each tree needs a minimum of 500 cu. ft. (15 cu. m.) and a space at least 7 ft. (2 m.) wide for small trees and 15 ft. (4 m.) wide for large trees. The length should be as long as possible to achieve a mature, healthy tree. Whenever possible the tree pits are connected together so the root space can be shared by all the trees.
Water Treatment
The whole area is also used for stormwater containment. Drainage basins sit in the top of stone/soil mix so water running off the surface pavement is able to flow down, through the stone and into the root zone of the trees. As the water flows through the stone/soil mix it is used by the trees. Any surplus water is emptied into the original storm drain for the street. The catch basins in the new construction are built with a metal screen near the bottom where oxygen can also enter the soil/stone mix. The oxygen is absorbed by the tree roots and prevents stagnation of the water. Catch basins beside the street curbs and building gutters also provide water to the tree. Fortunately, salt used on the streets in winter does not seem to cause a problem in the soil or with the tree roots because of the high volume of water flowing through the stone/soil mix.
Dealing with Trees
New trees of the right size and species to fit the planned dimensions and location should be delivered to the site and installed in the conventional manner within the concrete box. New trees are generally large, balled and burlapped, and installed directly on top of the stone base and in contact with the soil filled pores. If the existing trees are to be retained, the soil should be washed off the roots of the tree. The roots are then covered with a small size stone near the trunk, that gradually increase in size to meet the large stone installed in the trench. Soil is washed into this stone as it was in the rest of the installation trench.
Like every urban tree installation, several factors must be considered before actually installing the tree.
- Carefully select the species based on the city – Right Tree
- Trees should be installed where conditions, site location, and soils are suitable for the tree – Right Place
- Select a tree to improve local environment as well as collect and treat stormwater – Right Reason
Since the Stockholm Solution began, over 2,000 projects have been completed within the city, and the Swedish national government has accepted this concept for all future projects in the country. This program resulted in tree growth that is equal to or better than trees growing in nearby parks. It seems that all trees, regardless of species have thrived in this program. Tree experts from nurseries and other cities all remark at how well the trees in Stockholm look. Their color is dark green and leaf count is much higher than expected for trees growing in a typical urban environment.
Additional information:
- YouTube collection of photos. https://www.youtube.com/watch?v=S7kbSnnJwDI
- YouTube Lecture given by Bjorn Embren. https://www.youtube.com/watch?v=MojlKaAKeh8
- Handbook providing construction drawings and detailed information on the construction process by Bjorn Embren titled “Planting Beds in the City of Stockholm, A Handbook”, published by the City of Stockholm in 2009. This free book requires Adobe Reader and will take a couple of minutes to upload.