Roger: 0:02

Talking Trees with Lillie and Jad. Welcome to Talking Trees. Today we explore the design and construction of tree pits using structural soils, focusing on sustainability and healthy tree growth in urban environments. We'll dive into the benefits of using materials like gravel and biochar, which support sidewalk load bearing while allowing water and air to reach the roots. We'll also touch on how biochar improves water quality and nutrient retention. Plus, stockholm's tree planting guide offers practical advice for ensuring proper installation, drainage and root protection. Let's dig in.

Jad: 0:44

Hey fellow arborists, you ready to geek out a little on some urban forestry? Today We've got a whole stack of resources all about optimizing tree growth in those tough urban spaces.

Lilly: 0:55

Absolutely. We're talking healthy canopies, even when space is limited.

Jad: 0:59

Exactly, and we've got some great material to dig into today.

Lilly: 1:02

We've got Ben Rose's article on LinkedIn that he wrote about the structural soil method that they're using in Stockholm.

Jad: 1:09

Okay.

Lilly: 1:09

We also have Stockholm's handbook called Plant Beds in Stockholm City.

Jad: 1:13

Nice.

Lilly: 1:13

And we've also got a practice note from the Stockholm Tree Pits, which is a great resource for arborists.

Jad: 1:18

Perfect. So much to explore. I know you've had a chance to review these sources. What jumped out at you?

Lilly: 1:28

Well, the Stockholm Handbook really paints a picture of just how challenging it is for trees to grow in a city. You know they list limited space, compacted soil, competition with utilities, salt damage, all these things impacting root development and ultimately, how healthy the tree can be.

Jad: 1:41

And it all comes back to those root systems, doesn't it it?

Lilly: 1:43

really does, and that's something that Handbook does such a great job of highlighting.

Jad: 1:47

Yeah, it's fascinating how they describe these pipeline roots that develop in urban trees.

Lilly: 1:52

Yeah.

Jad: 1:53

Almost like they're adapting to find any little bit of space and resources they can.

Lilly: 1:57

Exactly. They have to snake around all these obstacles underground, becoming the tree's lifeline.

Jad: 2:03

It's amazing how resilient they are, but those pipeline routes can also spell trouble if they don't have room to grow right.

Lilly: 2:09

Absolutely. They can start causing damage to sidewalks, pavement, underground infrastructure. It becomes a real issue.

Jad: 2:15

Which brings us to structural soils. Exactly this is where structural soils have so much potential. I was reading Ben Rose's article and he's really highlighting that potential. Yeah, providing that stable foundation that we need for pavements and sidewalks, but at the same time, giving roots the space they need to thrive.

Lilly: 2:32

Exactly, it's like engineering the perfect underground environment for them.

Jad: 2:36

So you're essentially creating this sort of custom-built root zone, but how does that actually translate to real-world benefits?

Lilly: 2:43

Well, for one thing, it means you can have larger tree pits even in those really tight urban spaces, which means you can actually grow larger, healthier trees.

Jad: 2:52

Yeah, Does it actually prevent that soil compaction over time?

Lilly: 2:55

That's one of the biggest advantages. The way it works is you have this compacted stone base that acts like a shield, almost preventing the soil above from being compressed.

Jad: 3:04

Oh.

Lilly: 3:04

Even with all that foot traffic and vehicle loads.

Jad: 3:07

So the soil stays loose and airy.

Lilly: 3:09

Exactly, and because the roots can grow deeper, you get less of that surface rooting.

Jad: 3:14

Which is what causes so much pavement damage.

Lilly: 3:16

Exactly so less root damage, healthier trees, fewer sidewalk repairs it's a win-win for everybody.

Jad: 3:22

Definitely. But let's be realistic. Cost is always a factor in these projects. Are these structural soils actually feasible from a budget standpoint?

Lilly: 3:30

That's what's so surprising. They can actually be more cost effective than those modular crate systems in a lot of cases.

Jad: 3:36

Oh really.

Lilly: 3:37

Yeah, the materials are easy to get and the installation process is pretty straightforward.

Jad: 3:43

So we're talking like crushed granite, basalt, recycled concrete, that kind of thing for the stone base.

Lilly: 3:49

Exactly. It's all about using a well-graded aggregate mix to create all those voids for root growth.

Jad: 3:56

Okay, Now the Stockholm method takes all this a step further with their specific approach.

Lilly: 4:01

Yeah.

Jad: 4:01

They're really emphasizing those quality materials in that all-important aeration layer.

Lilly: 4:06

It's all about the details. With the Stockholm method, they specify using crushed granite or basalt for that base layer. Okay, because it's durable and it has good pH stability. They also really like using recycled concrete.

Jad: 4:20

Right.

Lilly: 4:21

Because it's a more sustainable option.

Jad: 4:22

It aligns with those sustainability goals. And they're very specific about the sizing of those aggregates Generally 32 to 63 millimeters for that base layer. On top of that baser, yeah, you've got that 200 millimeter aeration layer that they talk about in the handbook.

Lilly: 4:36

That's critical. It's a layer of clean 20 to 40 millimeter stone and that layer is what allows air and water to move throughout the whole profile.

Jad: 4:46

I see it's like the lungs of the system, yeah exactly.

Lilly: 4:49

They're also very clear about what not to use.

Jad: 4:51

Oh really.

Lilly: 4:52

No samgras, which is that unsorted crushed rock. No fines in these systems.

Jad: 4:58

Gotcha, because those fines would clog up the voids and defeat the whole purpose.

Lilly: 5:01

Exactly. It's all about paying attention to the details with this. So what else stood out to you about the Stockholm method? Well, they recommend using a specific soil type, type B, which has low clay and organic content, because it flows easily into that structural layer, ensuring that all those voids get filled with a good growing medium.

Jad: 5:20

So it's really about that perfect balance of materials.

Lilly: 5:23

Right and they also emphasize carefully placing that tree pit foundation to prevent settling and root intrusion into the pavement.

Jad: 5:32

So thinking long term to avoid issues down the road.

Lilly: 5:35

Absolutely. There was one thing that really surprised me.

Jad: 5:38

What's that?

Lilly: 5:39

They really push for test excavations before any work begins.

Jad: 5:43

Really.

Lilly: 5:43

Yeah, you really need to understand the existing root distribution and soil conditions before you start designing a structural soil system.

Jad: 5:51

That makes sense. Get a clear picture of the site before you start digging.

Lilly: 5:54

Exactly, it's like a soil health checkup.

Jad: 5:57

I like that Now both Ben Rose and the Stockholm Handbook talk about the use of biochar in these systems. Yeah, it seems like it's gaining a lot of traction in urban forestry these days.

Lilly: 6:08

Biochar is like charcoal, but it's made from organic waste. Okay, it's got incredible water holding capacity, ooh yeah, which is essential for trees in those hot urban environments.

Jad: 6:20

Absolutely. It's like a desert out there for them sometimes, and it also enhances nutrient availability and improves soil structure. So like a supercharged soil amendment.

Lilly: 6:28

You could say that, but here's the kicker it decomposes extremely slowly.

Jad: 6:34

Oh wow.

Lilly: 6:35

So the benefits last much longer than traditional amendments like peat moss and from a sustainability standpoint it's a home run. Yeah, Biochar actually sequesters carbon from the atmosphere.

Jad: 6:48

Wow, that's incredible.

Lilly: 6:49

So it's a win-win for the trees and the environment.

Jad: 6:51

It's amazing how something as simple as biochar can have such a profound impact.

Lilly: 6:55

It really is and Stockholm is really embracing it.

Jad: 6:58

Yeah.

Lilly: 6:58

They're actively promoting biochar use and they're even producing it locally from garden waste.

Jad: 7:04

Oh, wow.

Lilly: 7:05

Yeah, Now you know, there's always that debate between structural soils and those modular crate systems.

Jad: 7:12

Right. Both have their pros and cons.

Lilly: 7:14

The Stockholm Tree Pits practice note points out some really interesting advantages of structural soils.

Jad: 7:19

Okay, I'm all ears.

Lilly: 7:21

Especially when it comes to versatility. Well, for one thing, those crates are really rigid in terms of their geometry, so it can be really tricky to adapt them to existing underground structure.

Jad: 7:32

Oh, I see.

Lilly: 7:33

And dismantling them for maintenance can be a real headache.

Jad: 7:36

Yeah, that makes sense.

Lilly: 7:37

Not to mention the fact that they're made of plastic.

Jad: 7:39

Right, not exactly the most environmentally friendly option, exactly. So where do structural soils have the edge?

Lilly: 7:46

Well, the practice note highlights that they're much more adaptable. You can install them around existing underground features, which is a huge advantage in a dense urban setting. Plus, they're much easier to excavate if you need to do any maintenance. And let's not forget the sustainability factor. Using recycled materials for those structural soils aligns much better with our goals as arborists.

Jad: 8:06

Absolutely. Now, one thing that stands out to me is that aeration layer that we talked about. It seems like that's a major advantage over those older crate systems that didn't always provide adequate aeration.

Lilly: 8:17

Modern structural soil designs have really addressed that issue, making sure that those roots get the oxygen they need to thrive.

Jad: 8:24

And what about cost? I know we talked about structural soils sometimes being more cost effective. Is that also true when you compare them to those older crate systems?

Lilly: 8:32

It can be. The materials for structural soils tend to be readily available, and often they're less expensive than those prefabricated crates.

Jad: 8:39

Interesting, so there are both environmental and economic benefits to consider.

Lilly: 8:44

Absolutely. It's all about finding the right solution for each specific site and situation.

Jad: 8:50

Well, this has been a great overview of structural soils and the Stockholm method. What else did you find interesting in that Stockholm handbook?

Lilly: 8:58

Well, they actually take this structural soil concept a step further with something called biochar macadam.

Jad: 9:03

Biochar macadam.

Lilly: 9:04

Okay.

Jad: 9:05

Okay, how is that different from a typical structural soil mix?

Lilly: 9:08

It's like they took the structural soil idea and supercharged it. Okay With biochar. They incorporate a whole bunch of biochar directly into that macadam.

Jad: 9:16

So it's boosting all those benefits we were talking about earlier.

Lilly: 9:18

Exactly that water holding capacity, nutrient enhancement, all that good stuff.

Jad: 9:23

It's like you're optimizing the entire growing environment, not just creating those voids for the roots.

Lilly: 9:29

Exactly. What's really interesting is that the handbook mentions some promising results from their experiments with biochar macadam.

Jad: 9:37

Oh really.

Lilly: 9:38

Particularly in terms of tree growth and drought tolerance.

Jad: 9:42

That makes sense when you think about all the benefits that biochar brings to the table.

Lilly: 9:46

Right, it's like creating this incredibly resilient growing environment.

Jad: 9:50

Especially in those harsh urban conditions.

Lilly: 9:52

Exactly.

Jad: 9:53

Now let's shift gears a bit and talk about stormwater management.

Lilly: 9:56

Okay.

Jad: 9:57

We all know that trees play a crucial role in mitigating stormwater runoff. Do these structural soil systems actually contribute to that?

Lilly: 10:06

Absolutely. The Stockholm Handbook has a whole section on stormwater management and how structural soils can be a part of the solution.

Jad: 10:13

I'm all ears.

Lilly: 10:14

Well, the biggest thing is infiltration. Those open voids in the structural soil allow rainwater to soak into the ground.

Jad: 10:22

Okay.

Lilly: 10:22

Instead of just running off into the storm drains.

Jad: 10:24

That's huge for reducing flooding and erosion.

Lilly: 10:26

Exactly, especially in those heavily paved areas where you get so much runoff.

Jad: 10:31

It's like creating a giant sponge underneath the city streets.

Lilly: 10:34

Yeah, a good way to think about it. And remember those aeration wells we talked about earlier. Yeah, those play a role in stormwater management too.

Jad: 10:41

How so.

Lilly: 10:42

They capture runoff from sidewalks and streets and direct it right into the tree pit.

Jad: 10:47

Oh, okay. So it's like a built-in irrigation system.

Lilly: 10:50

You could say that it's utilizing that rainwater instead of letting it go to waste.

Jad: 10:54

Very clever, yeah. Now a lot of us are working with trees that are already established. Maybe they're even struggling in less than ideal conditions.

Lilly: 11:00

Right.

Jad: 11:01

Does the handbook offer any guidance on revitalizing those older trees?

Lilly: 11:05

They do. They have a whole section on what they call plant bed renovation.

Jad: 11:08

Oh nice, A tree spa day.

Lilly: 11:11

Exactly. It's all about giving those older trees a boost. What does that involve? Well, the first step is always. A thorough assessment Makes sense. You've got to know what you're dealing with Exactly. They talk about doing test excavations, soil analysis, looking at the root system.

Jad: 11:27

Okay.

Lilly: 11:28

Gathering all the information before you start making any changes.

Jad: 11:31

So once you have a clear picture, what's next?

Lilly: 11:34

The goal is to create a more hospitable growing environment for those existing roots.

Jad: 11:39

Right. Give them a fresh start.

Lilly: 11:40

Exactly. This might involve carefully excavating around the tree to expose some of the root system, and then you backfill with either structural soil or biochar macadam.

Jad: 11:52

I see, so it's kind of like giving those roots a second chance.

Lilly: 11:55

You could say that Now remember those pipeline roots. We talked about Yep. They need some special attention during renovation.

Jad: 12:00

Okay, how so?

Lilly: 12:02

The handbook recommends covering those popeline roots with a layer of macadam, usually like two to six millimeters.

Jad: 12:08

Okay.

Lilly: 12:08

And then a good amount of that nutrient-enriched biochar and compost mix.

Jad: 12:13

So you get a little extra TLC.

Lilly: 12:15

Exactly those roots are critical for the tree's survival, so you want to treat them with care.

Jad: 12:20

So we've talked about the concept of structural soils, different materials, the biochar, mechanic, but let's get into the nitty-gritty of actually installing these systems.

Lilly: 12:29

Right, let's talk logistics.

Jad: 12:30

Does the Stockholm Handbook offer any insights on the installation process?

Lilly: 12:36

They really do. They go into a lot of detail about material selection, installation techniques, common mistakes to avoid. It's a treasure trove of practical advice.

Jad: 12:46

Okay, give me some highlights.

Lilly: 12:48

Well, one thing they really hammer home is the importance of using the right aggregate size.

Jad: 12:52

Okay.

Lilly: 12:53

And avoiding any materials with fines that could clog up those voids.

Jad: 12:57

So no SAM cross allowed.

Lilly: 12:58

Exactly that. Unsorted crushed rock is a recipe for disaster.

Jad: 13:03

Because it'll mess up the drainage and aeration.

Lilly: 13:05

Exactly. It's all about making sure that water and air can move freely through the system.

Jad: 13:10

Okay, what about compacting that stone base? Any tips on that?

Lilly: 13:14

The handbook stresses the importance of compacting in layers to create a really stable foundation, and when it comes to adding the soil mix, they recommend flushing it in with a strong jet of water. Yeah, it helps ensure that all those voids get completely filled.

Jad: 13:29

Interesting. So there's a real art to this installation process.

Lilly: 13:31

There is. It's not just about dumping a bunch of rocks and dirt in a hole.

Jad: 13:35

What about those tree pit foundations? Any advice on how to construct those?

Lilly: 13:38

They recommend either pre-sabricated concrete foundations or building custom ones using granite edge supports or concrete pillars. Okay, the key is to have a really solid structure.

Jad: 13:49

Right, so it doesn't settle over time.

Lilly: 13:50

Exactly.

Jad: 13:51

Yeah.

Lilly: 13:51

And you don't want those fines from the pavement base migrating into the structural soil layer.

Jad: 13:56

Makes sense. Everything has its place.

Lilly: 13:58

Right, where do you think those aeration wells should go?

Roger: 14:01

Hmm, make sense, everything has its place, right.

Jad: 14:02

Where do you think those aeration wells should go At the lowest points in the tree pit, I'm guessing.

Lilly: 14:06

You got it that way. They can collect the most water Okay, and they won't become overflow points. It's also important to make sure that aeration layer covers the whole perforated section of the well.

Jad: 14:16

Okay, so air and water can move through freely.

Lilly: 14:19

Exactly Now. What about the geotextile?

Jad: 14:22

Oh right, when does that fit into all of this?

Lilly: 14:24

That goes on top of the aeration layer.

Jad: 14:26

Okay.

Lilly: 14:27

And it acts as a barrier.

Jad: 14:29

A barrier against what.

Lilly: 14:30

It prevents those fine particles from the pavement base from clogging up the aeration layer and the voids in the structural soil.

Jad: 14:38

Okay, so it's like a protective layer, exactly, yeah.

Lilly: 14:40

A simple but essential element.

Jad: 14:43

So no skimping on the geotextile.

Lilly: 14:45

Nope, it's worth its weight in gold.

Jad: 14:47

Okay, so we've covered a lot of ground here. It sounds like the Stockholm Handbook provides a really comprehensive guide to installation.

Lilly: 14:56

It does. But even with all that information, I imagine there's still a learning curve when it comes to actually putting these techniques into practice.

Jad: 15:04

Right, there's always that gap between theory and practice.

Lilly: 15:07

But after digging into these resources, I'm feeling a lot more confident about tackling a structural soil project.

Jad: 15:13

I hear you. It's great to have that practical guidance.

Lilly: 15:16

And what's really helpful is that the handbook actually includes a checklist. Oh really Of all the key stages in the construction process.

Jad: 15:23

Oh, wow, so you don't miss anything.

Lilly: 15:25

Exactly. It's like having a virtual supervisor right there with you.

Jad: 15:28

That's fantastic. Now we've talked about construction and installation, but what about the long-term care of these systems?

Lilly: 15:35

Right Maintenance is key.

Jad: 15:37

Does the Stockholm Handbook offer any insights on that?

Lilly: 15:40

They do. They have a whole section on maintenance and monitoring and they really emphasize the importance of regular inspections.

Jad: 15:48

To catch any problems early on.

Lilly: 15:49

Exactly. You need to be checking those aeration walls for clogs, looking for signs of soil compaction or surface rooting, making sure those tree pit foundations are holding up over time.

Jad: 16:01

So it's all about being observant.

Lilly: 16:02

And catching those issues before they become major problems.

Jad: 16:05

What else do they recommend for ongoing maintenance?

Lilly: 16:07

They stress the importance of supplemental irrigation, especially during those first few years after planting. Those young trees are still establishing the root systems.

Jad: 16:16

Right.

Lilly: 16:16

So they may need a little extra help to get through those dry periods.

Jad: 16:21

And the structural soils can actually help with that.

Lilly: 16:23

Absolutely. Those voids act like little reservoirs, making the irrigation more efficient.

Jad: 16:28

It's all connected.

Lilly: 16:29

It is.

Jad: 16:30

Yeah.

Lilly: 16:30

Let's talk about something that no arborist likes to think about, but it's a reality.

Jad: 16:35

What's that? Tree damage? Yeah, accidents happen. Construction projects go wrong.

Lilly: 16:40

It's unfortunate, but it's part of the job.

Jad: 16:43

Does the Stockholm Handbook address this at all?

Lilly: 16:46

They do. They have a whole section on assessing and penalizing damages to trees.

Jad: 16:51

Wow, yeah, they take it seriously.

Lilly: 16:53

Very seriously.

Jad: 16:54

I'm curious about their approach. What happens when a tree is damaged?

Lilly: 16:59

They have this standardized system for assessing the extent of the damage.

Jad: 17:03

Okay.

Lilly: 17:03

They look at things like the size and health of the tree, the type of damage, how it might impact the tree's long-term survival.

Jad: 17:10

So they're not messing around.

Lilly: 17:12

Nope. They also factor in the tree's location and its overall contribution to the urban forest.

Jad: 17:17

It's like they're giving each tree a personalized evaluation.

Lilly: 17:20

They are and they don't hesitate to issue penalties to those responsible.

Jad: 17:24

It's about accountability.

Lilly: 17:25

Exactly. People need to understand that damaging trees has consequences.

Jad: 17:31

But what happens if a tree is damaged beyond repair?

Lilly: 17:34

The handbook outlines procedures for tree removal and replacement.

Jad: 17:37

Okay.

Lilly: 17:38

And they emphasize the importance of doing it responsibly and sustainably.

Jad: 17:41

Makes sense. You can't just replace a mature tree with a sapling and call it a day.

Lilly: 17:45

Right, they actually provide guidelines for calculating the replacement value of a tree.

Jad: 17:50

Oh, wow.

Lilly: 17:51

Taking into account its size, species, age and its contribution to the ecosystem.

Jad: 17:56

So they're recognizing the true value of these trees.

Lilly: 17:58

They are. They understand that trees are more than just decorations. They provide so many benefits.

Jad: 18:04

Shade, stormwater mitigation, air purification, aesthetic beauty.

Lilly: 18:09

Exactly so. Removing a tree should always be a last resort.

Jad: 18:12

And when it does happen, it needs to be done right, absolutely.

Lilly: 18:16

It's inspiring to see a city like Stockholm taking such a holistic approach to urban forestry.

Jad: 18:21

I agree. They're really setting an example for other cities to follow.

Lilly: 18:24

They're showing that it's possible to create these thriving urban forests even in challenging environments.

Jad: 18:29

And they're sharing their knowledge so we can all learn from their experience.

Lilly: 18:32

Absolutely. Their website is a fantastic resource. I highly recommend checking it out.

Jad: 18:37

This has been a fascinating dive into the world of structural soils and the Stockholm method.

Lilly: 18:42

I've learned so much, you too.

Jad: 18:44

And I'm feeling really inspired to try some of these techniques in my own work.

Lilly: 18:47

It's exciting to see all these innovative solutions emerging.

Jad: 18:50

It is. It feels like we're really starting to make progress in creating healthier and more resilient urban forests.

Lilly: 18:56

It's a great time to be an arborist.

Jad: 18:58

I think so too. Well, on that note, we'll wrap things up for today.

Lilly: 19:02

Okay.

Jad: 19:02

To all you arborists out there keep up the great work, keep those canopies thriving and remember there's always more to learn. So keep exploring, keep innovating and let's create a greener future for our cities together.

Roger: 19:19

Thank you for joining us on this episode of Talking Trees. Today we explored the design and construction of tree pits using structural soils to support healthy tree growth in urban environments, From the benefits of biochar to Stockholm's guidelines for installation and root protection. We hope these insights inspire better tree planting practices in your city. Until next time, stay rooted in sustainability.