How to use biochar for structured soil plant beds in urban areas
The carbon negative, easy and nature-based method for growing stronger city trees. Getting started with biochar experiments in urban environments is easy, affordable and low-maintenance.
“The Stockholm model”: Learn about one city’s approach to cutting out urban tree death
Biochar is known to improve soil health. Biochar retains nutrients in the soil, reduces water consumption and improves soil tilth, thus providing a natural, organic method for soil regeneration and plant resilience. Given its useful properties and proven track record in natural soils, biochar has been of growing interest to urban forests and built soils. This past decade has produced promising results on the benefits of urban biochar programs.
Biochar works equally well in urban soils and natural soils, boosting the city plant’s soil bedding and creating stronger, more resilient urban trees. This is a simple, carbon negative technology that you’ll want in your next urban green project!
But getting started with biochar as a part of a city tree maintenance routine can be tricky. When to add biochar? What type of biochar is best for which type of city soil? How to mix biochar and other organic soil material for maximum results?
In this report, you will learn
- What Stockholm, Sweden discovered in their decade-long experimentation with biochar
- The right way to build structured soil tree beds and after-care
- How to get started with biochar / urban tree pilots
Why city trees?
Planting new city trees is not enough, more needs to be done for trees to thrive in challenging urban environments
Climate resilient cities rely on trees to improve air quality, cooling and shading, and to mitigate surface water flooding. Trees are also an important carbon sink — an averaged-sized tree can store hundreds of pounds of CO2 over its lifetime. Across the research community, the message is that we need more urban planting to realise the climate change mitigation potential of trees. At the same time, new methods and techniques are needed to ensure that the trees that do get planted stay healthy and thrive.
Because urban trees are typically nested in urban roads and walkways, they have limited soil space to work with and often struggle from the start. What’s more, constant traffic and heavy machinery can significantly compact the soil, turning it as hard as concrete and preventing the soil and plant beds from breathing; trees grow on soil that is so dense that the roots can’t move.
The city of Stockholm, Sweden is a global pioneer in restoring their dwindling urban canopy using organic biochar soil packs and an innovative method to construct tree beds.
The Stockholm Biochar Project
From a tree die-off rate of two-thirds to a 100% success rate with biochar
The city of Stockholm has been experimenting with biochar for the last decade in response to an alarming rate of new tree die-off: In 2009 when the program started, two-thirds of all city trees were either dying or dead. The city’s program to incorporate biochar into the soil has stopped tree death in the city (no new tree die-off reports since the program start!) and in 2017, excited by the promising results, the city scaled the program to all its urban greeneries and trees, and is planning to build more biochar production units around the city.
Since Stockholm has adopted this methodology, not a single newly-planted tree has died. This is especially encouraging, as trees are most often at risk of dying during the first years after planting.
A resounding success, the program has enabled
- Cost-savings in city maintenance
- Circular woody waste processing
- Stronger, greener trees
One of the most interesting learnings from the program has been about how to build strong city tree plant beds using structured soil and biochar.
Structured soils — mimicking a naturally aerated soil
The city of Stockholm proposes a new way of layering organic materials to mimic natural soil conditions and promote a better soil structure for challenging urban conditions, captured below in the diagram.
In a natural soil environment, the plant’s roots co-exist with stones, bedrock and other root systems. Similarly, the idea behind a well-constructed, structural urban plant bed is to enable more natural growing conditions in city soils. Structural soil plant beds enable water and nutrients to better enter the soil. Holes in the soil created by gravel and biochar enable oxygen and CO2 exchange. The sponge-like biochar is a key component in this free-flowing ecosystem, absorbing water, air and nutrients.
In the Stockholm plant bed model, large rocks from construction sites are compacted to create plant beds for city trees. Mattias Gustafsson, who worked on the project, notes that rocks worked well for Stockholm as they had plenty available, but the material could be something else, as long as it doesn’t compact the soil and is locally available. The city uses a mix of gravel / crushed rock, biochar and compost (called ‘fertilized biochar’ in the diagram) to make a stable structure. Finally, the plant bed is covered with a hard surface, and is thus stable enough to withstand heavy traffic without compacting the soil.
Stockholm now plants all new trees with the method and has begun to restore their old trees as well. If you are new to the practice, the best time to take up structured soil and biochar programming is when new trees are planted, old trees are being removed, or street repairs are about to be done. This allows for the construction of the structured soil from scratch.
Structured soils work best where trees are given ample room to grow. The exact size of the plant bed depends on the plant type, however it’s always good to leave room for growth. Depending on the tree’s surroundings, we estimate that a plant bed will need between 0.5–2 tons of biochar. With a biochar density of 250 kg/m3, Stockholm uses about 0.5 tons of biochar per tree.
In urban parks or peri-urban areas, soil health and needs differ from cramped city conditions. For example, in Stockholm’s more spacious suburban areas, the city used gravel, biochar, and compost in a 6–1–1 mix as plant bed building blocks, instead of the above-presented structured soil.
Easy after-care that fits into pre-existing routines
Biochar is a natural, stable, solid, and safe material that, once planted on the soil, can be left to do its magic for decades or centuries to come. This means you don’t need to reapply, change or otherwise treat the biochar in plant beds.
And the after-care is easy: After building the structured soil plant bed, add a simple Nitrogen Phosphorus Potassium (NPK) 0.01 % solution water to the trees over the next two years, and follow the same care process as you normally would. You can use any type of fertilizer for this treatment. The after-care and early years’ maintenance is done to ensure that the soil gets all the micronutrients it needs to grow strong, green trees for years to come.
Stockholm’s experiment provides us with convincing results. The three key lessons to remember when designing a structured soil plant bed model for your community are:
1) Fertilize your biochar with compost. This allows for biochar to perform its best.
2) The soil needs room to breathe. Using upcycled materials like gravel can be a powerful way to elevate the soil.
3) Every context is unique. Even with the above tips, it’s worth noting however that there are many ways to plant a street tree, and you know the best how your city or urban area should build its greeneries.
How to get started with urban tree pilots
At Carbo Culture, we’ve authored a guide on using biochar for city trees. Head here to download our guide, Planting Urban Trees with Biochar.
This year, we’re working with select cities, urban planners and environmental NGOs in California to develop urban tree pilots and pre-pilot testing. During the pilot, we provide biochar for small scale tree planting (5 to 20 trees in one year) and advise on project activities.
In 2021, we’ll be working with city pilots in Northern Europe.
Will you plant new urban trees, remove and replace tree die-off, or have upcoming street repairs in the pipeline for 2020/2021? Then, you might be a great candidate for our city pilots! Send us a message describing your current tree challenges: trees(a)carboculture.com
Have you used biochar and structured soils in your urban greenery? We’d love to hear about your experiences and possibly interview you for a podcast or a blog collaboration. You might also consider joining our CarboCollective community to learn about activities near you.
We thank Mattias Gustafsson and Lotta Ek from EcoTopic as well as Björn Embrén and everyone from the Stockholm Biochar Project in providing their insights and expertise as the basis for this article.
