Urban trees are indis­pens­able for healthy air in cities. However, they are also exposed to special stress factors and there­fore have sig­nif­i­cant­ly shorter lifes­pans and higher main­te­nance costs. For this reason, the Swedish capital Stock­holm has been testing special soil sub­strates for city trees during the last 10 years. These sub­strates contain gravel, clay, sand, lava and, with increas­ing success: biochar.


Urban trees suffer above all from soil sealing and soil com­paction in urban areas. This severe­ly limits the avail­abil­i­ty of water and oxygen for the tree roots.

First measure: Stone layer

The first step in Stock­holm was to replace the highly com­pact­ed soil around the trees with a layer of stone. Stones do not con­dense to the same extent as earth. This allows water to con­tin­u­ous­ly pen­e­trate between the tree roots, leaving enough space for the exchange of gases. In the early years, two sep­a­rate stone layers with dif­fer­ent stone sizes were stacked up. However, it turned out that the best results could be achieved with a uniform layer and a stone size of 32–63 mm.

Second measure: Biochar

In the begin­ning, the Stock­holm team poured soil between the stones to provide the trees with an addi­tion­al growth medium. The arborists increas­ing­ly replaced the soil with biochar, as plant char­coal resists com­pres­sion and com­paction just like the ground stones. Togeth­er with the ground stones, the biochar improves the void frac­tion, also called poros­i­ty. A higher poros­i­ty pro­motes gas exchange and water reten­tion capac­i­ty, more per­me­abil­i­ty leads to improved root pen­e­tra­tion. Biochar is not only much more porous than sand or clay, it is also not biode­grad­ed or com­pact­ed as quickly as peat, for example.
1–10 mm proved to be the optimum par­ti­cle size for biochar, and around 2.25 m³ of biochar per tree has also proven itself in Stock­holm. This results in a mixing ratio of 85% gravel and 15% biochar per tree, while a mixing ratio of 75% gravel and 25% biochar proved to be optimal for shrubs.

Better rain­wa­ter management

Another addi­tion­al benefit of the plant coal-stone sub­strate was improved rain­wa­ter man­age­ment. Through a special design of the sur­round­ing street pave­ment, the accu­mu­lat­ed rain­wa­ter is direct­ed from the roofs, pave­ments and streets to the trees. The high water storage capac­i­ty of biochar and its fil­ter­ing effect not only reduce costs and efforts for tree care, but have also achieved cost savings in urban water management.

Storage of an addi­tion­al 2,000 t CO2

If Stock­holm were to use biochar for all its green areas and land­scape projects, the author esti­mates the annual demand for biochar would be around 800 tons. This would have the addi­tion­al benefit that an addi­tion­al 2,000 tonnes of CO2 from the atmos­phere could be bound in the soil each year.

Orig­i­nal article: Plant­i­ng Urban Trees with Biochar. The Stock­holm Project.
Author: Björn Embrén
Pub­lished in: The Biochar Journal (tBJ) 2016, Ithaka Insti­tute, p. 44–47