How does the future of urban greeneries look like?

Adopting biochar and structural soils in tree planting helps cities build healthy, green and resilient tree covers that survive.

Resilient cities and city tree infrastructure are central to responding to changing climate conditions, but urban environments put tree and plant life under increasing pressure. The City of Stockholm, capital city of Sweden, has been using biochar in all of its urban plant beds for a decade now. The results? Dramatic reduction in tree deaths, healthier, stronger trees and more beautiful plantings.

Let me share why we at Carbo Culture started working with urban greeneries and wrote an open-source guide to learn how to introduce biochar to plant beds in urban environments.

More than half the world’s population lives in urban areas. From flooding to extreme heat, many cities are already living in the midsts of the effects of the changing climate. Cities are at the forefront of the fight against climate change with the ability to adapt quickly and transform into greener living. Around the world, cities of all sizes have pledged to seek solutions and take action to curb emissions.

A strong urban tree canopy, or a thriving city tree population, is an evidence based method to make urban infrastructure more climate change resilient. City trees are vital to air quality, public health and safety, they provide cooling and shade on hot days, increase property values and improve health.

In larger cities, urban trees bring health and resilience benefits that are worth millions of dollars annually, and canopy coverages are valued as an important infrastructure asset. In New York City alone, the positive economic impact of NYC trees is estimated to be $120 M a year. Yet our city trees are often not surviving the urban life, because the lack of oxygen and space combined with the non-natural environment leave many trees dying and in need of replacement. In some cities, up to a quarter of all trees need to be replaced.

Cities are tough environments for trees to thrive. Urban trees often grow in poor conditions under concrete pavements in compacted soil and with poor gas exchange to the roots. In many urban areas, such as San Francisco, existing trees are dying at the same rate new trees are being planted. The expectation for new tree survival is low.

If we can improve the wellbeing of city trees, we can both improve the air quality in the city and help to make the concrete jungles more liveable. Thriving trees have even a positive mental affect on citizens.

Greening our cities and storing some carbon

Carbo Culture is a carbontech startup native to the San Francisco Bay Area. We’re fighting climate change by converting local Californian biowaste, such as walnut shells, into high quality, stable biocarbon called biochar — a building block of regenerative agriculture and soil health. Each ton of biochar stores the equivalent carbon of three tons of carbon dioxide, that won’t escape for hundreds of years.

As we studied the science of biochar further, we noticed that it can play a big role in helping soil in cities as well.

Stockholm, the capital city of Sweden has been using biochar in all its new urban tree infrastructure for the past decade. Dubbed the “Stockholm model”, the city has developed a new method of using structured soils and biochar to plant their trees. This has led to impressive results in keeping trees growing and healthy, not to mention the array of benefits to citizens’ health and the city maintenance budget.

Biochar is to the soil what coral reefs are to the sea — structure and surface area to foster life in.

Biochar acts like a sponge for the soil. The structure of biochar helps retain nutrients and water, and brings new surface area for the microbial life to thrive in. Did you know that a gram of biochar can have over 3000 sq ft of surface area!

As shown by the decade long experience in Stockholm, biochar improves the soil utilised in urban plant beds by :

• Cultivating soil micro-organisms needing access to carbon, hydrogen, nitrogen, phosphorus and oxygen
• Biochar can substitute finite resources often used in plant beds, like sand, peat and clay
• Improving stormwater filtration and retention — securing a water supply and creating a water reservoir in the soil, and preventing stormwater flooding with better drainage

How Stockholm got its trees back with biochar

The Stockholm model is a method to build and remake urban plant beds. By using large rocks from construction sites, Stockholm creates plant beds with space to allow roots, water, air and nutrients to spread. A mix of gravel, biochar and compost is flushed down with water into the stable structure creating a long term supply of nutrients for the plant. The structured soils alleviate the compaction and promote air flow to roots, while the biochar acts as a home for microbial soil life to thrive in.

The development of the plant beds in Stockholm got started in 2009, when the city noticed that all of their newly planted hard surface trees were dying. The use of structural soils with biochar in new and restored plant beds started as a trial to improve the conditions of urban trees growing in compacted soils. The result has been healthier, stronger trees and more beautiful plantings. The positive results became the start of the Stockholm Biochar Project, a city wide rollout of the biochar efforts on all city trees, which has been hugely successful. Stockholm even won the Bloomberg Philanthropies’ Mayor’s Challenge in 2014 with the Biochar project. Structural soils with biochar is now the standard for how all public plant beds in the City of Stockholm are built.

Biochar pilots in California cities and counties in 2020

Working with the people that pioneered the Stockholm model, we wrote an open guide to help all cities, towns and counties get started on using biochar for their city trees. In 2020, Carbo Culture will start new pilots of supplying biochar to urban green roofs and cities in California.

We make our biochar from agricultural residues such as walnut shells in California’s Central Valley. With our production technology, we’re sure to make conductive, high surface area, consistent and clean biochar — which has been made with a carbon negative, beneficial total environmental footprint.

As any grand project, nothing would ever happen without the change maker who starts to think differently. Stockholm city was fortunate enough to have the innovative arborist Björn Embren, who took action and tried this new method to help the trees survive, with great success!

Since Stockholm has adopted this methodology, not a single newly planted tree has died. This is especially encouraging, as newly planted trees are most at risk during the first years after planting.

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 for this guide.

EcoTopic is an environmental consultancy that has helped Stockholm and other cities in starting their biochar use in planting city trees.

Contact us at trees@carboculture.com! We look forward to seeing lots of more cities adopting these tools to their benefit.

Thanks for reading. -Henrietta, Charlotta and team