Evaluation of planting beds

The City of Stockholm constructs planting beds that delay and purify stormwater, contribute to recreational and social values, and increase biodiversity.

In Stockholm Royal Seaport, the design of planting beds has been further developed, primarily with different plant substrates and plants that retain, delay, and purify stormwater. There is considerable interest in scaling up the Stockholm Royal Seaport concept for planting beds in the rest of Stockholm. However, to scale up these solutions, the functionality of planting beds needs to be further evaluated.

Purpose and objectives

The project aims to design a full-scale trial to test stormwater treatment and delay in planting beds intended for tree planting.

The purpose is to design a programme to test planting beds and their capacity to treat and delay stormwater, as well as evaluate the planting beds used by the city. The project also intends to propose alternatives to the city's planting beds and to design a statistically relevant full-scale test in which, among other things, planting bed construction and substrate selection can be evaluated concerning treatment and delay of stormwater.

Results and experiences

Early in the dialogue with the Stockholm City Development Administration, it was decided that StormTac Web, a model for water management in urban environments, would be used as the primary source for stormwater treatment and pollutant occurrence in stormwater. The model indicates the treatment capacity that can be expected from different types of stormwater facilities and for various pollutants.

Using biochar in the substrate is now well-established in Stockholm and given the climate benefits and positive tree growth results, it seems justified to find a way to combine this with stormwater treatment. Biochar can contribute to both treatment and nutrient leakage, so using the right type of biochar is crucial. Carbon macadam seems to have such high permeability that it does not contribute to treatment through filtration, and some modification of the system may be necessary. However, this theoretical study alone is not sufficient to draw conclusions about carbon macadam. Therefore, it seems relevant to conduct a full-scale test over as long a period as possible.

To determine whether there are other substrates relevant for testing in the full-scale trial, planting bed manuals and standard drawings from four other Swedish municipalities have been studied. Several substrates and various ways to design planting beds have been discussed. The final proposals include six different planting bed designs, two from the City of Stockholm's planting bed handbook, one from Stockholm Royal Seaport, and the other three are new variants inspired by these three.

Planned activities

The City of Stockholm plans to conduct a full-scale test of different planting bed designs. Funding will be sought from the City's climate investment funds.