Episode 556 Trees in Stockholm

Bjorn Embren was the man in charge of renewing Stockholm’s urban forest — especially the trees along roads and other public places where the soil. was seriously compacted. He learned at a conference in Hannover that porous rocks can provide a hospitable environment for tree roots. He began to line the bottom of trenches with heavy stones and above them a mixture of small pebbles and biochar. The heavy stones allowed for enough stability for traffic. He add water with nutrients for a couple of years and ensures that there is opportunity for oxygen to reach the roots. He uses the city’s waste as a source of biochar, which helps clean the water from streets that would otherwise contaminate the city’s waterways and the Baltic seas. The healthy urban forests have impressed other Nordic countries to emulate Stockholm’s innovation. For the video, audio podcast, transcript, summary, and comments: https://tosavetheworld.ca/episode-556-trees-in-stockholm.

Bjorn Embren

Todd Irvine

Robin Collins


trees, biochar, soil, Stockholm, stone, water, cities, ground, years, compacted, plant, root, material, mixture, construction, beds, nutrients, grow, Bjorn, planting


Metta Spencer, Todd Irvine, Robin Collins, Bjorn Embren


In this conversation, Metta Spencer, Bjorn Embren, Todd Irvine, and Robin Collins discuss the innovative approach to urban forestry in Stockholm, Sweden. Bjorn Embren led the development of an extensive tree planting project throughout Stockholm. The project was inspired by research from the University of Hanover, which found that the best root development for trees occurred in open stone material.

In Stockholm, they used large stones (macadam) to create tree planting beds with high porosity that could support any load, such as buses and trucks. These beds allow for efficient gas exchange and water infiltration, making it ideal for tree growth. To capture rainwater for the trees, they built wells (or cisterns) that collected water from roofs, sidewalks, and streets, reducing pressure on the sewer system. This innovative approach to urban forestry has been adopted by other cities in Scandinavia and has been praised for its benefits in counteracting flooding and promoting healthier tree growth in urban environments.

Bjorn Embren described Stockholm’s innovative planting method, which involved using biochar, a form of charcoal, to enhance soil quality. Initially, the city used a mix of compacted stone and soil to support tree growth, but the introduction of biochar provided exceptional results. Biochar improved the water and nutrient retention capacity of the soil, which allowed trees to grow faster and healthier than ever before.

The success of this method was partially due to the use of water bags with nutrients that provided a constant supply of both water and nutrients to the trees. It was discovered that the biochar could store excess water and nutrients, ensuring the trees never experienced shortages. This method has been tested and refined over the years to create optimal planting conditions for trees.

The city was introduced to the Bloomberg Philanthropies competition for future cities, where their innovative use of biochar earned them second place. The prize money was used to build their first biochar machine, which processed garden waste from Stockholm residents into biochar. This process engaged citizens in the fight against climate change.

Using biochar in urban tree planting not only helped with stormwater infiltration and soil quality, but also cleaned polluted water from roads before it entered nearby lakes, such as Lake Mälaren, which provides drinking water for Stockholm residents. This technique offers multiple environmental benefits and highlights the importance of innovative methods in addressing urban challenges.

While urban tree planting may not significantly reduce CO2 levels in the atmosphere, it provides valuable services such as air purification and water pollution control. By incorporating biochar and other innovations into urban tree planting, cities can work towards a cleaner, more sustainable future.

The participants discuss the effectiveness of structural soil for tree growth in urban environments. Structural Soil is a medium that can be compacted to pavement design and installation requirements while permitting root growth. It is a mixture of gap-graded gravels and soil. It provides an integrated, root penetrable, high strength pavement system that shifts design away from individual tree pits. Todd Irvine remarks that structural soil works well initially, but over time, soil volume may be lost. Bjorn Embren argues that this is not the case, as trees in urban areas can survive for a long time without additional nutrients. Trees take energy from the sun and transfer it to the ground, supporting life within the soil. A self-supporting system is created, similar to nature. Structural soil allows for better water infiltration and gas exchange, essential for tree survival in urban settings.

The ideal soil for tree growth should have both mineral and humus components. Structural soil uses larger fractions of material, such as recycled concrete and local materials, to keep the ground open and prevent compaction. In addition, biochar can be added to soil to improve its quality.

In regenerative farming, soil should not be disturbed, as this can cause CO2 to escape. However, oxygen must still reach the roots of plants. Bjorn explains that water infiltration helps oxygen reach the roots without disturbing the soil. Crushing stones for backyard use may not be necessary if the soil is not stressed. However, using soil with biochar can improve its quality.

When constructing new parks in urban areas, heavy machinery often compacts the soil, making it difficult for trees to grow. Structural soil can help alleviate this issue. Over the past 20 years, Stockholm has planted around 40,000 trees using this method. While some tree species may react differently to the process, new trees generally do not show any difference when planted in structural soil.       

Bjorn’s innovative method of planting trees in cities involves using a mixture of biochar, compost, and stones to create a more resilient and water-absorbing planting bed for trees.

He explained that the typical construction for planting beds is one meter deep, with the first 60-80 centimeters being the planting bed. By connecting the planting beds of multiple trees along a block, it allows the root systems to travel from one side of the block to the other, thus providing access to water and nutrients. This method also helps to increase the surface area for mycorrhizae, leading to healthier trees.

Bjorn mentioned that the limiting factor for tree growth in Stockholm is usually water availability, but once every 5-10 years, there may be too much water. In such situations, it is important to select tree species that can withstand a lot of water. The use of biochar and compost in the planting mix has proven effective in improving the water absorption capacity of the planting bed.

Bjorn criticized the use of plastic boxes in the ground as a tree planting method, arguing that it is environmentally catastrophic and doesn’t consider the long-term implications for urban areas. He believes that trees can grow well in stone materials and that using open stone material mixtures can help protect against flooding in cities.

In response to Metta Spencer’s concerns about the high mortality rate of newly planted trees in Canada, Bjorn suggested that involving local communities in tree planting and maintenance could help improve tree survival rates. By making individuals or families responsible for the care of specific trees, it could lead to better tree care and foster a sense of community.

Bjorn Embren discusses a project where 20,000 cherry trees were planted in biochar and compost in small ditches. The trees were healthy and grew well, even without additional water or nutrients after the first year. Metta Spencer expresses her hope that this approach could influence tree planting in Canada, as the environment is similar to that of Sweden. Embren states that they have been successful with this method for over 20 years, and he believes the gas exchange and water infiltration are important factors for success.


The following transcript has been machine generated using “otter.ai.” Prior to using information from the transcript, please watch the video to catch any obvious errors.

Metta Spencer  00:00

Hi, I’m Metta Spencer, today, let’s go to Stockholm. Would that be fun for you? I haven’t been to Stockholm in maybe 20 years, but we are going to look at some trees, or at least think about trees and talk about trees. and we have somebody here in Toronto who’s also a tree man, and so we’re going to talk about Toronto trees, as well as the innovative approach that we’re going to mostly consider which has been adopted by the Swedes in Stockholm. So, this is Bjorn Embren.

Bjorn Embren  00:35

[Inaudible] waves.

Metta Spencer  00:35

He is the man who was in charge of developing this very extensive planting of trees throughout, throughout Stockholm that kind of made, made waves because people in other parts of Scandinavia got so enthusiastic that they’ve adopted his methods and have also planted trees in some of the other major cities of the North. So in here in Toronto with me is Todd Irvine, who is an arborist he’s a man who actually goes out and cuts limbs and prunes and fertilizes and teach and takes people on walks and, and does seem to individually acquaint us with trees. So good morning, Todd, how are you?

Todd Irvine  01:28

Good. Thank you.

Metta Spencer  01:29

Good to meet you. and in Ottawa is a Pugwashites friend. This is Robin Collins, who is a member, a very important member of the Pugwash community. and we’ve been holding conversations about urban forestry for several months now. With the idea that we’re going to ask the Pugwash group to lobby the Canadian government to promote the use of urban forestry, in, among other things, as a way of handling our increasingly hot climate, cities. So I became more aware of the Stockholm thing several months ago, when I was talking to a guy Albert Bates in the US who is a big enthusiast for bio char and he was mentioning the work that Bjorn did in planting on a large scale, planting trees throughout Stockholm. and I’d like for you to just give us a run through if you can Bjorn about what you did that was so special in, in Stockholm.

Bjorn Embren  02:46

The, the basic about Stockholm solution is using have shifted stones, we call it macadam in Sweden, and it’s assorted stone material that we started to use with 90 to 110 millimeters big stones that we put down in the ground. and it all started with the visit University in Hanover, where they have tested different kinds of soils and, and the plant bed constructions and trying to see where the root system developed at the best way. And after 10 years, they excavated the, these all these test beds and realized that it was in the open stone material that they got the best root developments. So it’s really long time research work that they did, they mesh it all the root system and the stems and the leaves and everything to see where they got the best results. So even if they had the best compost, compost soils that you can think of, all the different kinds they could get ahold of in Germany, it was always the stone mix that they got the best results from. So I had visited them in the university and got the lecture. and after that, I went back to Stockholm and tried to convince the all the hardcore civil engineers in the traffic department in Stockholm that this was the best thing you ever could do. It [inaudible] to get the trees to grow. and of course, they shook their head and said no way you’re going to make it.  So, but since then, things happened as it always does for me, I [inaudible] and look down in excavation shaft. and so something at the bottom of that shaft that was big, solid rock stones put on top of each other, and it looks exactly what the Germans had talked about in Hanover University. So I went back to traffic department and of the civil engineers what was in that excavation. and they suddenly shined up and looked extremely happy and said oh, that’s the best way of building streets. That’s what we have done the best streets in Stockholm for the last 100 years. and so that was the opening for the whole project in Stockholm that I’ve passed that excavation, so the stones in the bottom and could explain so they could connect to this solution, and that it was the same thing as when they been droughts. So that gave me the first opportunity to test it, because they, when they were so happy about talking about the solution. I, I was, couldn’t I do some tests anyway? Yes. So of course you can, and then they let me do it in the sidewalk. Yes, to try it. So that was the opening of the whole thing. So that they research in the Hanover is the basic of all things that we do in Stockholm now about the root system developments, and or the first 10 years we were doing it with, with this kind of big stones select the stone material that we put down in the hole in the, for the plant bed and compacted. So, we got to strong construction that can hold for any traffic, you can think of, buses, trucks everything they can take any loads we want. and still you have 35 to 40% of porosity in the material. So, you can’t destroy the porosity in this plant beds, it’s impossible. And it will stay like that for at least 100 years, because it’s the same construction for trams and for trains and everything all around the world really. And when they actually do excavation is how can we sometimes find old tram construction in the ground, and it’s made of these kind of big stones. and the trees there’s a standard close to it always shows that material before the compacted soil things that’s on the other side. So, it always shows this open material, because the gas exchange is working, and the infiltration of water is much easier and the road system can travel along this stone construction of the old tram or railway tracks. So, we really see things that show us that it works for a long time this way of construction. So that is basic, stones put on top of stones compacted and then you can begin the close surface with the asphalt or stone’s or concrete or everything. But the problem that appeared for me in Stockholm was that the civil engineers didn’t want to have an open space or open materials so we could infiltrate water and get the gas exchange to work. So they have to do, had to use a well to infiltrate stormwater or rainwater and to get the gas exchange to work in our plant paths. And, and so we’ve made it, had a local blacksmith to make a well and it’s became a standard in Stockholm to use that kind of well in hard surface and the water.

Metta Spencer  08:49

I want to I want to question you because I think I’ve seen a picture of that. What I saw was you had dug a trench alongside the street, and you put these big rocks in there and then covered it with I don’t remember what but it wasn’t dirt. It was like a compost or biochar.

Bjorn Embren  09:10

Biochar, yeah,.

Metta Spencer  09:11

And every so often in between, you know, every, not every tree, but every few trees.

Bjorn Embren  09:17

Yeah, every second tree.

Metta Spencer  09:19

A container looks like a container that people come and put water in that container. and then…

Bjorn Embren  09:29

It is just rainwater, is just rainwater.

Metta Spencer  09:31

It’s rainwater, and then it goes from there down into

Bjorn Embren  09:36

Yeah, it’s like a layer of clean stones on the top, like infiltration layer, that the gas exchange works is long for the planting bed and the infiltration layer. So when it comes a lot of rain, it’s yeah, it can take a lot of rain. It’s really a good way of counteracting flooding in cities and things like that so. Yeah, and then the rain also…

Metta Spencer  10:01

Excuse me, when you call that a well, I would call a cistern. If you’re actually capturing the water, you’re not bringing water up from the subsoil, you are, you’re bringing you’re capturing the rainwater. So, okay, well anyway, it will It then flows along this trench.

Bjorn Embren  10:23

Yeah, try to collect as much water as we can Stockholm is rather dry, we got 600 millimeters of rain every year. So it’s a dry city. and so we really need to catch as much water as we can. So we take roof water from the buildings around the sidewalks and from the sidewalk and the streets. So we collect street water, sidewalks, water and roof water. So we really unloads the pressure on the sewer system really, a lot. and so the water company were had a hard time to understand how much water we took away. and because we take away a lot of stormwater with these kinds of [inaudible].

Metta Spencer  11:12

Okay, now, tell me about the biochar part,…

Bjorn Embren  11:15

yeah, yeah, first, the first 10 years, we were using ordinary soil, we flush down with compacted stone, and then we flush down the soil between the stone, so we didn’t get any compaction of the soil. So that was the whole thing about this construction that the compact this stone, so everyone was happy about how strong it was as a construction, and then they flush down soil between the stones. and you could only fill it up to 25% of the porosity. So, you’re always had a lot of open space between the stones together with the soils. And after 10 years, we made some experiments and I wanted to dry charcoal for the barbecue charcoal, just to keep humidity or water holding capacity high on the surface when we played some tests with the just clean stone baths, and we got such a good results for the grass when they had this 50% charcoal in the, the soil mixture for the grass. So, we tried it on the other side of the street for trees. So, we took 50% soil 50% barbecue charcoal, and mix that together, and we got results that we hadn’t seen ever before, not even in the nurseries they could get such good results. and we, we were just shocked about the effect. Afterwards I can explain why it happened because we’re using these kinds of water bags. You know then when you water trays with bags, these water bags that keep 75 litres of water and you put them around the tree, so it slowly penetrates the soil with water slowly for over eight hours. And so we used that for that that kind of trees and they had this mixture of 50% biochar and compost and in normal case, it would have been a big mistake because so much biochar or charcoal in the ground together with soil, it would wouldn’t have been enough with nutrients in it. It would, couldn’t grow because the biochar would take away all the nutrients from the soil. So the trees shouldn’t get in it but we use the, the, these water bags with nutrients to ppm parts per million of nutrients in these water bags and they got 75 yeah 75 liters every week for the trees. So, they grow. They had constant supply of water constant supply of nutrients and if it was some left of the nutrients or some left of the water, it will store be stored in the biochar. So, it wasn’t any shortage of water or nutrients at any time for the first two years when we had planted these trees. That’s why we got that the extreme effect from the first time so we were sheer luck because if you had to use that water bags with nutrients, it should have been a real bad outfall. But this is how things happen in the world sometimes it’s sheer luck. So, after that, we started to think about what we could do with biochar and the years to follow was really testing, testing, testing different mixes. They have different kinds of for nutrients, with the biochar, and everything like that, and then and we came up with the idea that we should start to produce our own biochar because we couldn’t get ahold of enough volumes of charcoal, it was charcoal at that time we couldn’t get ahold of in a biochar in the beginning, so we couldn’t get ahold of it. So, we started by from Finland and from Germany, just to be able to be in plant beds with biochar and make Wait, did they make theirs from city waste dumps or what? Yeah, so that’s what all right, this we presented it for the politicians, and they said, okay, gave us okay to make, once you call it yes, some, some paper about our thoughts about how this could be applied in Stockholm. But before that paper was ready, we got this question if we wanted to participate in the Bloomberg Philanthropies competition, about future cities, and I said, yes, of course, there was six different things that were competing in Stockholm, and we got the, we got to represent Stockholm, in, in this competition.

Metta Spencer  16:35

So what was the whole thing of that? Bloomberg was giving….

Bjorn Embren  16:39

How you can make future cities. work, what you say?

Metta Spencer  16:46

I think it is called C 40 cities. and former Mayor David Miller is, was the head of it at one point, and now he’s starting an institute for them so that they’re running information to share with all cities around the world, they offered a prize to cities that came up with innovative suggestions.

Bjorn Embren  17:11

Okay, so we got the second prize, I was extremely disappointed. But after a while, it was okay. The money was enough for us to build our first biochar machine. So it worked for a couple of years. and the winning concept was that we took care of the garden waste from the citizen of Stockholm that came with it to the recycle centers, and they got biochar back when the left the garden waste at the Recycle Center. and so it was that was the winning concept that you get the people in Stockholm to participate in the the climate fight so to say. Okay, yeah, yeah, it has been a fantastic year to be in and, and it’s, it’s so great to be a part in something that’s really good for the environment. I mean, in, in so many levels, when it started, make plant beds like this, you infiltrate stormwater, and you use material in the plant bed that clean the storm water from you, take care of the nutrients, clean some of the pollution from the roads, roads, and everything. Instead of that it goes straight out to the lakes around Stockholm, or, or the Baltic Sea and things like that. So it’s really, it’s one thing after the other that just say, this is the right way, you can’t make it better than this. It’s Yes. It’s so great to be part of it.

Metta Spencer  19:00

Well, that’s, that’s wonderful. and I think we have all more or less concluded that planting trees in the city will not very, very rapidly do much to reduce co2 from the atmosphere. It’s not going to make a big dent in global warming, but it will do a lot of other services for us such as cleaning the air and I hadn’t thought about cleaning the water so that it doesn’t pollute the Great Lakes and things like that.

Bjorn Embren  19:29

Now for Stockholm it is so extremely important because the Lake of Mälaren that Stockholm is separated by is the our drinking water that we use for drinking water is not for all people in Stockholm drinking the water from that lake. and we are letting our polluted water from the streets straight out of it. So it’s extremely important that we can take care of it before so we put it into the ground and let all the life in the ground, take care of the, the pollution and nutrients that come with it.

Todd Irvine  20:07

I guess what you’re talking about a structural soil is that correct? Like, so is the size of the stone that you’re using?

Bjorn Embren  20:16

In the first 10 years, we were using 90 to 110 millimeters. It’s like a big ball of like this a handball, or what’s it called, it’s like this, and that’s was because of what the civil engineers were so happy about the construction, that the soil in the excavation, so I have, okay, I take that size, so they feel comfortable but with what I’m doing, I must find ways to get through the system from the beginning. So, I had to get people pleased that works with construction. So that’s why we had the big size from the beginning. But over the years, we, we found out that we test it was accurate, same strength of the construction for you 32 to 63 millimeters or 32 to 90 millimeters. and we could pre mix it if we kept the, the mixture of biochar and compost, at most 15% in the mixture, it wouldn’t take, make any difference to the stability in the stone, mixture so you can put that next to stones and five, fifteen percent of Biochar and compost and compacted really hard and you will still have open material and you don’t destroy the construction with that low amount of biochar and compost. So that’s I would say it’s perhaps 90% of all the plant beds that build now in hard surface are made of these smaller fraction of those 32 to 63, 32 to 90 millimeters instead and it goes so much faster if you have premix and can compact it. So, half time, compared to the old way where we flushed downs between the stones and stuff.

Todd Irvine  22:22

And do you find like I mean, what I’ve read about structural soil is that it works very well for the first number of years, but at some point, you lose soil volume, because there’s no

Bjorn Embren  22:32

That’s, that’s just clean bullshit. I mean, I promise you, if you go to any city in the world, and look at the old past the old trees that have been standing there for 100, year or more, no one is giving them any nutrients or anything, and they will survive for an extremely long time. And what’s the great thing about trees is that they take energy from the sun and put it into the ground through the root system. and they work together with all the life in the ground. and they grow and they die, and they grow and they die, root system and all the bacteria’s and the insects and the worms, everyone will grow and die. And they feeds from the sun through the trees. So you get much, you the extent the, the organic material in the stone mixture increases, it don’t decreases, it increases it’s exactly the same, that the same thing that’s happened out in the nature, same thing. You have life in the ground, and you have to support the life in the ground with water and energy from the trees. and the gas exchange is one of the absolute most important thing, to get oxygen into the ground. It’s for the Mycorrhizae? need oxygen and all the insects and the bacteria everything that support trees and, and works together with the trees need oxygen and water in the ground. And that’s what’s the really success about this thing. It’s about the stone the structure that are, are kept over the years and the growth of the humus part in the ground. So it’s a self-supporting system. It’s like nature, its nature.

Todd Irvine  24:36

Mass forests are usually primarily soil. Yes, there’s forests with stones, but I think it certainly makes sense in this application. But I don’t want listeners to then think that all their backyard should be big piles of stones. I mean yes, when you’re trying to bear loads of trucks and you want compaction and you’re concerned about the loads, absolutely. But I think our goal should still be when possible, to be having soil volume. and just soil with humus layer and mulch.

Bjorn Embren  25:14

Within this, this is what it is it’s a mineral pot, and its humus pot. It’s exactly as soil is built up. But it’s a, it’s just bigger fraction. and it gives you opportunity to take loads. and it gives you an opportunity to don’t get compacted because that’s the biggest problem people have in the city as to how to keep the ground open for the water to infiltrate instead of go out to the sewer system. and to have the gas exchange to work. If you have a big park, and you have these all this what you call it happening, happenings, you have music and you have sunbathing and you have these lawn mowers running all over the place all the time, and you compact ,compact, ordinary soil. Extremely, they stressed soil all the time, and you have to put the open up the soil every third year or something to get the grass to grow and things like that. So, it’s really an easy solution to use these kinds of selected stone material to keep the ground open. and sometimes after a while is you if you have it on grass or in a different plantation, the, the surface can get compacted after some years, but you will always have it open in the ground if you use this kind of materials. and what it is, is waste its waste material that we’re using, leftover materials, the garden waste biochar from garden waste and stones, recycled concrete recycle breaks, recycle piles, and the local material that you can use instead of like it is in Sweden using sand, you’re using clay and use using PET peat moss. and that is is letting co2 out in there. If you keep pet Moss from the ground, you will let the carbon dioxide go out in the atmosphere and that…

Metta Spencer  27:30

I’m a little confused. I am interested in what people talk about regenerative farming, one of the things they talk about is do not plow do not turn the soil over. Because if you do that you’re gonna let the seal or the carbon in the soil escape as co2 in the air. and that’s what we want to keep from happening. But is that incompatible with what you said? About you want to have the gases coming into the oxygen coming into the roots and so on. How are you gonna get the oxygen in there without having the co2 escape?

Bjorn Embren  28:08

Good question. I mean you have a living soil and it’s working like any soil from the beginning, and I don’t have an answer all of it. But you’re, you’re not turning the soil around. You’re not killing the, the life that so you get this co2 to just escape because decaying material. I think it’s more like a closed system in the ground if you don’t move it around. It’s the same in my aquarium. I have biochar in my aquarium, and I have a {inaudible], when you see the layers of the glass, you can see how different layers have different kinds of life. So, you and you shouldn’t move around it because then the life that’s up here, gets to the ground to the bottom and then they die and then you will lose a lot of I think it’s the same thing that if you disturb the soil, you will get a lot of organism that will die and that co2 Go. I think it is like that.

Metta Spencer  29:25

How do you get the oxygen in? You said the gases. You wouldn’t get it down deep into the roots right? How do you get there without turning over the soil?

Bjorn Embren  29:37

The same as in nature, water goes down. So if you have water to fill up the planting bed and then it slowly sinks to down in the ground, and then it takes the oxygen with it. First it press out the carbon or the bad gases. and when it sinks, it takes new oxygen down. So that’s how it works out in nature. And it’s the same thing with all [inaudible].

Robin Collins  30:03

If I could ask a question, I just want to drill down a little bit with on, I want Todd’s question, why wouldn’t people crush stone for their backyards instead of having soil as they normally do? So? So, a couple of questions here. One, is what you’re proposing good for only certain kinds of vegetation and trees? And, the I think the supposition is that there’s more nutrients within soil than there is with a mixture of material plus stone. Aside from what nutrients the stone, the crushed stone might provide, if it, you know calcium and so on, or whatever the kind of stone it is. So, I mean, that’s another question. What kind of stone are we talking about here? But there does seem to be a, a confusion that comes to my mind, which is, if this is such a great project, and I mean, it sounds good. I mean, you’ve been doing it for decades. But so what’s the what’s the downside?

Bjorn Embren  31:14

If you have soil and you don’t have any stress on the soil, use old soils. and but anyway, you can always put biochar in soil and you will get better soil than you ever had before. That’s basic, but you should use when you can use, you should use soil, when you can use soil when you don’t have the stress moments. I mean, you must remember, because when you build new parks in big cities around the world, have you seen the machines that work with the soils and put out the soils, and they’re 25 tonnes weighed on the machines, and they do the excavation, and they run around with this 25 tonne machines on the ground where the root system in the future should penetrate. And then they put some industrial made soil on top of it and put it out with this, yeah, this machinery. What do you call this thing that you put out soil with?

Todd Irvine  32:22

A grader

Bjorn Embren  32:24


Todd Irvine  32:24

Does it grade?

Bjorn Embren  32:28

The thing that lift out this oil, they use the same have the Iron thing to compile it to finish the surface and the pressure they put on the surface with that on the soil is just destroying the soils. And the soil that you buy from industrial soil you buy is dead from the beginning. So it can’t recover the first year because there is no life in the soil that you buy. So the soil content, recreate the aggregate in the soil structure, and if you have compacted from the beginning, and you yeah. So that’s a big problem. and it’s really hard to get things growing when you use big machinery in that way.

Metta Spencer  33:21

And tell me how many trees you planted. I read someplace Stockholm planted something like 40,000 trees in this operation? Is that right?

Bjorn Embren  33:31

Yeah, at least Yeah, it is. I mean, I think for this 20 years, you could calculate something like 1000 year, 1000 trees every year, over the whole period of a little bit more than 20 years. So it’s a lot of trees that we have been planting, and we have made mistakes, and try to learn from the mistakes. and that’s why we made this handbook to try to really show people how to make it and to give decision makers the knowledge of what you can do with this kind of construction.

Robin Collins  34:15

Are some trees better for this project and others? I mean, I mean in terms of root shallowness?

Bjorn Embren  34:23

Not, not if it’s new trees, then it’s no difference between trees. But if you are re rebuilding, planting beds around the old trees that are 100 years old, they react differently, and it’s a little bit how the structure was before if they really have a circular root system that can’t expand anymore and, and especially Maples Acer Serie is really a problem to when they’re extreme old. But Oak trees, Celia’s, Langtry, and things are exploding. If you give them a new plant and like this, if they’re 50-100 years old, or something like that, and been standing still for 50 years, you can restart them immediately.

Robin Collins  35:20

How deep how deep is, is, are the layers, I mean. How deep is the deepest?

Bjorn Embren  35:28

I mean, the ordinary construction they make is one meter deep, three feet, three feet deep. And then the, the, the first feet is the construction for the surface. So it’s around 60 to 80 centimeters planting bed, but they always connected as big, as many as possible. So along the block, it’s usually 10 to 12 trees connected to each other. So the root system can travel from one side of the block to the other side, if it’s more water, and that that part or the block, and the mycorrhizae? can infiltrate everywhere, and you get so much surface area, on the sides and on the bottom of this kind of of solution. So the trays will always find what they need. The limiting factor is always the water and in Stockholm it is always too little water and but once every 5-10 years, we get to places where it’s too much water and then you have to select your crease that can stand a lot of water. That’s my first carrot in biochar stone makes it’s a after why when they had made this for hard surface, we realized that it was the same problem with ordinary soil. So we started to use this as ordinary soil instead. And then the size of stone was two to six millimeters or four to eight millimeters. and we have 25% of biochar and compost in the mixers. So I took away some of the soil in my, my vegetable garden and started to grow carrots and potatoes and things in this misc mixture. So this is how it looks in most cities in Europe, you have concrete slabs, and some kind of stone slabs or asphalt on top. And then you have a setting stance asphalt. and a bearing layer of crushed stones, 0 to 63 millimeter, heavily compacted. And this is the problem you can’t get the gas exchangeable, and you can’t infiltrate stone or water rainwater in this construction. That’s the basic problems. and this is how it likes; it looks by excavating own place with trees that been standing and haven’t grown for anything over perhaps 20 or 50 years or something like that. And it’s just mixed material, nothing to do with a good planting bed. So the trees are finding some cracks in the ground and put out the roots that support the whole trees, one root could support a whole tree, and they’re so sensitive to excavation when they look like this. So this is an ordinary site in Stockholm, if you take a sidewalk and excavated you will find these kinds of cables and pipes and everything. And that’s been put down there over the last 100-150 years and so it’s a problem to really find a way to build a plant bed but it’s it’s so adaptable to be different things in the ground. You just put out two stones compacted and doesn’t matter if it’s pipes or, or cables or anything or other construction. You just follow the open volume and put two stones there. So it’s no really problem with installation in the ground. You can always find places to put out this little mixture, so this is what I talked about the big machine that destroys the ground. When you build parks, this is it’s always looks like this when big companies are building parks in Stockholm and out in Europe. They’re driving around with their tracks on the ground and then they put out the soil and compact it. So they have to in this space they had to remake all this construction because it wouldn’t grow afterwards. This is also common in parks in Stockholm they have this imbalance going on and a lot of people are running around, and this is re, after people have been using the ground, they have to replant it. and this is how it looks after six year if you don’t do any extra plant beds just tried to plant trees in ordinary compacted soil, it won’t work. And this is just to show how hard the material is when you have these ordinary soils that get compacted after a few years and so easily to understand that stormwater can’t penetrate this in the summer if it’s completely dry. and it comes, I have already suppose, yet we just run out in the street and go to the sewer system. So I try to pinpoint the 10 most important factors that are shown on this just to make it easier for people to understand what’s the important things you have to think of. You have to have a solution for all these 10 things. So this is the first time we’ve made these pre mix home with biochar. and it was before we use compost. So this is just biochar and mineral nutrients and stones. Pre-mixed, and it works well, too. So this…

Robin Collins  41:29

Do you find you don’t have to replenish the nutrients over time, but it’s a self sustaining process after a certain period.

Bjorn Embren  41:39

Yeah, it is. It is. So we support them with nutrients the first two or three years, and then we just leave them to their self. This is what the first the Stockholm basic solution is they have this big kind of stones in the bottom, and then they have the infiltration layer of the smaller stones. and they have the biochar compost mix in the bigger stones and nothing the upper part and you see that, what I call it the well or inlet for stormwater and the get the gas exchange to work. So that’s a close up to the, the the inlet and the how it’s placed in the infiltration layer. So this is the way it looks like on the surface of the we have saved the surface with concrete slabs. and you see they have this concave, concrete solution that collects as much water as possible and is connected to the, the pipes from the roofs so we get as much water as we can. And this is just outside Stockholm, a suburb, and it’s an open solution with a small kind of stone fraction instead, and we have direct infall of the water from the surrounding areas. And so you don’t need any, any solution for, for inlet so the plant beds in this way. You just get overflow from the sidewalks and from the streets in some places. So in this, you can plant anything you want. You can plant trees, perennials, shrubs or anything or flowers, whatever you want. So this is a example of this was planted the driest year ever in Stockholm, ever measured and still it established so fast. and this is don’t take it two years after the driest year ever. So it’s, it’s a good result, if you think about that. This is the basic mixture that gives compost biochar one part each, and six parts, so this is the basic if you want to try this. This is what you should use for the first time, later you can try your own mixes but start with this mixture. That’s the, that’s the basic thing to use. This is just to compare the difference in the soil on the left side and biochar compost stone mix on the right side. and this picture on the left side is taken three days after we made the computation of the of the material. It was the same kind of compacted compaction of the materials and the two tubes but of the three days, three days the water was still standing on top of the soil that was compacted but in the biochar compost stone, mixture it was, had penetrated this mixture in just a couple of minutes. It says take 3000 millimeters of rain water every hour. This is stone compost, biochar mixture. So it’s a it’s really a good mixture if you want to handle the flooding in the city. So this is what’s happening when you have this open material. So material, nothing can stop the root system to develop. So the dirt system is so massive, it’s so many roots that comes from the trees. and when you have so many roots out in the in there is so much air, the trees are not so sensitive to excavation. So if you excavate a root system that contains with small root system, in a healthy soil, it will recover immediately the first year, it is not a problem. But you remember the roots that I showed, from old planting beds where one root supports the whole tree, it’s easy to understand, if you take or break a root that support one root support one tree, that the tree could die. But if you had a root system with 1000s of small roots, then you can you don’t have the same problems.

Robin Collins  46:26

When you see when you see that that sort of cement square with the holes Todd. Do you? Do you think that some this would be viable for certain trees and not others? I mean, what about trees with very thick roots or older trees? Are they going to be able to penetrate down and around or something?

Todd Irvine  46:51

Well, I think you know, as Bjorn said, the, the reality is that the load is being borne by these stones, which means that the key thing is that there’s not a compaction of the pore spaces. and so there is room for the roots to get in between. You know, I think they’re the one limitation is that there’s much less space for roots. I mean, you need a lot more of this soil mix than you would if you didn’t do this, right, because it’s much of the area is stone. So there will never be room for roots in those areas. But the benefit, of course, is that you’re providing these pore spaces, which then allows for this soil in between those pore spaces. So yeah, I mean, I think it’s, it has a lot of benefits. Absolutely. and I don’t think the actual type of tree matters that much. It’s more that this mixture is correct, and there’s enough space between the stones, I still have concerns that over the long term that there wouldn’t be enough space just because so much of the space is stones in this case that’s like over 50% If not 75%, I don’t know Bjorn can speak to that. But we have problems here in Toronto where they use structural soil and then in about 15 years, there was no soil left because the roots and taken up all the void spaces. and so now they don’t use this for the most part they use silva cells…

Bjorn Embren  48:18

Plastic boxes in the ground, how could you excavate in a plastic boxes in the future? I don’t know. It’s so stupid for me. I mean, it’s so environmental, catastrophic thing to use, and plastic in the ground when we’re fighting climate change, and you start to talk about plastic in the ground in the city. I don’t know if you understand how a city works, how you build status, how you’re the, the city’s limit. I mean, it must be possible to excavate. You can’t have plastic boxes in the ground. I mean, it’s a massive approach from these companies to try to sell this plastic construction in the ground. It’s, it’s, for me, it’s just without any thinking. You don’t think about the future if you use plastic boxes in the ground.

Metta Spencer  49:18

Who does?

Bjorn Embren  49:20

They say, it’s a company that selling plastic boxes as a solution for whole surface in cities. And because they say you must have soil and I can show you how many picture as you want, that you don’t need ordinary soil for trees to grow. There is happily growing in stone materials everywhere around the world. And some of the best growing situations in China, in mountains of China. It’s just stones. So it’s ridiculous that people started say that it won’t work is the most extreme thing. If you really want to protect yourself from flooding, you have this open stone material mixers in the bottom. and it’s 50 to 60 centimeters down to the bottom. And if you infiltrate all this rain, water, or the sidewalks, water, everything into this one, and in the end, the water will go out to the Baltic Sea, when it has filtered through these plant beds. So this is also a solution, if you want, don’t want to have flooding in the cities. and this is really extreme, and it grows so fast and these kind of solutions in Stockholm. No, but I get, I get a bit upset when you start to talk about the company names in the in situations like this, because it tells me a lot of how strong they are in their propaganda about what you should use. and it’s always about money. And I’m happy because I have been working for the city of Stockholm. So I haven’t need to think about money all the time, I could just think the only thing I think about money is that Stockholm is quite rich compared to other cities in Sweden. So I share everything that we have developed, and what we have learned over the years, and we are the first to buy things and we buy it expensive, and then the price is goes down when we work with it for a couple of years, so everyone can use it.

Metta Spencer  51:43

You know, Canada has promised to to plant 2 billion trees, and they’re doing it. But my fear is they’re going to be planting these trees out on you know, long distances from where people are, and nobody is going to be watering them or putting nutrients or anything on them. and they and they’re gonna die. and my impression is that unless you really look after trees, the mortality rates quite high. I’ve seen some videos lately, places where people have been planting trees along the embankments of expressways and stuff and 70% of these newly, you know, one year old trees have already died. So my feeling is that we ought to be planting more of them in the city, and that we ought to, it’s not what you’re kind of approach because you have obviously big chain gangs of people who with equipment digging these trenches and with the technology, but I was thinking we should maybe get neighborhoods to, to be responsible for planting trees along their, their particular street or out we get plant trees along country roads. And where there are enough people so that individuals or families can say well that that’s a tree I planted and it’s up to me to make sure that it’s, it’s taken care of for the next few years. So I think the mortality rate might be better, and I think it would also be a very kind and general, general way of building community. People get acquainted with each other by going out with you know, and working in their, on their trees together. But of course, you’d need some ditch digging material and, and people bringing in compost and, and mixtures of your macadam and your biochar and your rocks and so on. So, tell me, and here’s where I want,…

Bjorn Embren  53:45

I can show you a project that we’ve made with 20,000 cherry trees, where we just planted them in biochar and compost in the small ditches.

Metta Spencer  53:59

Where’s Todd?

Robin Collins  54:00

He left.

Metta Spencer  54:04

He got angry?

Bjorn Embren  54:06


Metta Spencer  54:07

Do you think so?, Oh, I want to ask him his notion about how we would plant you know, how have a good urban forest in, in cities like Toronto, but actually all across Canada, and what is some notion of of the local situation, I’m sorry if he…

Robin Collins  54:29

I would, I would divide the the 2 billion extra Tree Project of the Canadian government from you know, the urban tree, I think, you know, there’s a good argument for urban trees, you know, as you were saying Metta that can be maintained by citizens and so on. But that’s the scale of that is is very small, slight compared to the 2 billion project which is really mass distribution in. Well, you know, you got like you say you’re going to lose some. But the survival rate of some trees in certain areas is actually pretty high from planting, like it’s 70 to 80%. Right. So,…

Bjorn Embren  55:16

But there is the machine and they made it. and then we refill it the biochar and compost and stone these ditches, and we made as long dishes as we could. and it was always along highways, and you never know what material is in the ground. So we dug this ditches, it was five meters apart, and then they planted to share trees every meter in the solution. And this is the the is small plants that they planted 80 centimeters in this stone mixture. And this is how it looked two years ago.

Metta Spencer  55:54

Hello, how old are those trees

Bjorn Embren  55:58


Metta Spencer  55:59

How old are those trees we’re seeing now?

Bjorn Embren  56:02

They’re after six years it looked like this.

Metta Spencer  56:05

Yeah. Wonderful.

Bjorn Embren  56:07

And they’re extremely healthy.

Robin Collins  56:09


Metta Spencer  56:10


Bjorn Embren  56:11

And no one has given them water in the last four or five years. It was just the first year they got extra water, and no extra nutrients, nothing.

Metta Spencer  56:22

Okay, well, this is really wonderful. I wish we could go on and talk a little longer, but and I hope that this has some influence on what we here in Canada decide to do, that I think that there’s a good case to be made for the value of planting more trees in cities. In general, and I don’t know, I mean, Canada is not that different from Sweden in terms of the environment. I presume that it’s an expensive thing to do. But you made your case, right, and other countries, Nordic countries are also copying you is that the case?

Bjorn Embren  57:05

It’s just so strange to hear about people that say that they stopped growing. I mean, we have done this for more than 20 years, and there’s still as new planted trees in their development. They’re still growing as they were newly planted after 20 years, and it’s really big trees that still are growing. So I don’t know what they did. If they didn’t have a good gas exchange, or the water mixture they were using too compact mixture or something like that. It’s something that’s not right. When they talk about structure and soil, and I think it’s the mixture that and the the gas exchange and the infiltration of water that don’t work in the right way.

Metta Spencer  57:51

Well, we can’t solve that right here. and now, I just want to thank you very much for this. I found fascinating adventure into an approach that I don’t think any of us have considered before. Right. Robin?

Robin Collins  58:10

Yes, very interesting.

Bjorn Embren  58:11

Yeah. and it’s interesting because it works in old cities, you can do it in old cities, not just in development areas. You can go everywhere and build it. Let’s go.

Metta Spencer  58:22

Thank you, thank you it’s been wonderful. Say Goodbye. Have a great Day.

Bjorn Embren  58:26

Bye Bye

Metta Spencer  58:33





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