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Expert Q&A: James Urban, FASLA, Urban Trees + Soils

When it comes to applying the science of urban forestry to design, few can match the expertise and reputation of landscape architect James Urban.

For more than 30 years, James Urban has specialized in the design of trees and soils in urban spaces. With a reputation as closely associated to the subject as his last name, he has contributed to the design of such notable city spaces as the Toronto Waterfront, the National Mall, and the plaza of the Hirshhorn Museum.

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Urban was instrumental in the development of structural cells (Silva Cells), structural planting soils, micro root paths, and macro soil trench techniques, and has been credited with helping to re-awaken the profession of landscape architecture to the skills required to successfully plant trees in difficult urban soils. In 2007 he was awarded the ASLA Medal of Excellence for this contribution the profession of Landscape Architecture. He is the 2013 recipient of the ISA Award of Achievement for long-term contributions to the profession of arboriculture.

In a video on your web site, you describe a shift in approach from “Put the right tree in the right place,” which was the design mantra in the late 1970s, to “Make the place right for the tree.” Is there a city or community that stands out as an excellent model of this approach?

Many cities are now starting to have minimum soil volume standards, which is what that statement is getting at. One of the best examples of a city that makes the place right for the tree is Toronto. They have worked hard over the last twenty years to improve the way they think about trees at all levels of government. They have done a fabulous job of changing their standards for the way they put trees in the ground.

Many of our readers may be involved in planning efforts aimed at increasing tree canopy in urban communities. What are the most important factors for planners to consider when embarking on an urban forest master plan, or when thinking about trees for a green infrastructure master plan?

If you’re just looking at tree canopy as a numeric, clearly the best thing you can do is start by saving the trees you already have. In my city, Annapolis, MD, we have an ambitious tree canopy goal of 40%, but we’re chopping down trees at an amazingly fast rate. State and city forest conservation legislation allows you to cut down trees and simply replace them with newly planted trees. In Maryland, if you cut down a forest tree, you are not required to put its replacement in a forest situation. It could be a street tree or in a parking lot island. There is also nothing in the entire law that requires you provide any soil or growing conditions for that tree, it is a very hollow set of rules. So even though we have a lofty tree canopy goal, we have probably gone in the opposite direction.

In other words, save what you can, but know that simply having laws or an ordinance in place isn’t going to help the effort if requirements for growing conditions are not included?

Yes. There should be some simple statement that the growing conditions for the trees you’re planting ought to be similar to the growing conditions of the tree you cut down. A tree in a forest not only has different soil and soil biology compared to a street tree, but it is part of a diverse ground plane ecosystem that includes multi-layers of shrubs and smaller trees. An ordinance that only looks at large trees—such as 6” DBH-is not helpful. You may replace that tree, but you have not replaced the damage you have done to that ecosystem.

What is the best way for planners and designers to factor in climate change when working to enhance urban tree canopy?

The goal of keeping a viable urban canopy is not so much impacted by climate change. Our trees just don’t live that long in the urban environment, unfortunately. We are constantly replacing them, so we can change species relatively rapidly. We also have the ability to react to things like emerald ash borer or Asian long horned beetle, which are more globalization-driven issues than climate-change driven. Our cities recovered from Dutch elm disease; we just planted different trees. Although, I should share this story: early in my career, I worked on a project where we replaced a huge planting of elms that had died on a campus in Connecticut with ash. We chose ash because it had a similar look and similar qualities. Ironically, those ash trees are now in the pathway of the Emerald ash borer.

We need another 20-23 new species to be available to designers and others who make decisions to plant trees so we can begin to protect ourselves with diversity. Most of those species will not be native. All of the native trees are already in use, or we aren’t using them because they don’t transplant that well into cities. There may be a way of breeding trees that can handle urban conditions, but after you heavily breed them for a particular quality, are they native? You may not like hearing this, but I’m not always a big fan of native trees. They are nice to use in cities, but I think the most important factor that makes a tree native to a particular location is the soil. You just should not be planting a tree in a soil type to which it is not native. If you do, that tree is no longer native; it’s exotic. I think we need to rethink our discussion about native trees when we are in the urban environment, and allow ourselves the privilege of using new trees that aren’t native because of global warming, globalization, and the way we have altered the soils in our cities. We need trees that are adaptable to the urban environment.

You mentioned the adaptability we have with urban trees, given their short life span. What is the average life span of an urban tree?

That is a loaded question which I think we shouldn’t even try to answer. The early numbers that were thrown out-7,9,10, 15 years-were not based on enough science. I have heard opinions from people I trust that 15-20 years might be a better number. But I don’t think we know, and I think we ought not to say “the average life of an urban tree.” We really should be looking at the average life of open grown trees in park areas, a parking lot tree, a street tree in downtown vs. a street tree in a one-story retail district, etc.

The other night my wife and I watched a documentary about a woman who was killed in a neighborhood in Queens, NY in 1964. The film included lots of photos of what the neighborhood looked like in 1964, and today. Being a tree guy, I noticed that the place was full of large trees that had been planted and had grown up in that neighborhood. This was in Queens, NY-not exactly the hotbed of good soils! I think we underrate the power of trees to grow in our urban environment.

Let’s talk about the design principles you present in your book Up By Roots. Four of the 10 principles are considered soil-based strategies. It seems critical, then, that the client and project team understand the importance of soil and its invisible components. What do you think is the best way to effectively educate clients and partners about the importance of soil to the overall success of the design?


I used to think that the best way to handle this would be to start suing landscape architects and designers when the tree dies, but not enough people care enough about trees to file a lawsuit. It must be education. The Sustainable SITES Initiative, which is now folded into LEED certification, makes a big step. I attended the first presentation of the outline for Sustainable SITES, and in the presentations, I observed that the word “soil” only appeared once, and it was only a subheading of a subheading. It was lip service to soil. To solve that, I got myself put on the committee. Now, 50 of the 200 SITES points you can earn are soil-related.


You also include three management-based strategies. The first is “Establish Reasonable Tree and Soil Budgets.” What is a common mistake designers make in this area?

Either the soil budget is not established early enough, or it is not reasonable. I have a project now with a very, very small budget for soil. I was brought onto the project team to design the soil, but I said, “With this budget, you don’t need me on the job, because there is nothing I can do.” That got the team’s attention, and I think we will get some more money, but it is expensive.

When I first started working on urban trees in the 1970s, the arborists had an old adage: “Put a $5 tree in a $50 hole.” That number is actually right. If I have a $500 tree, I need to put it in a $5000 hole. We need to spend a lot more money on the soil component. Now, if you are a homeowner on a single family, 1/8-acre lot in a city like mine (Annapolis, MD) you can probably plant any kind of tree and get away with it, but on Main Street in downtown, you probably can’t. It’s a matter of first understanding –assessing and analyzing—your urban soils, and then determining the best techniques to use in making meaningful changes to the soils to improve the problems you find with urban trees.

You also recommend “Creating Detailed Tree & Soil Construction Documents.” What are some of the newer and emerging specification tools that you’d recommend to planners, designers?

A couple of years ago, I was invited to be on a team with the Urban Tree Foundation in Visalia, CA. They had a grant to rewrite the tree planting and soil specifications for the state of California. We got the client to agree to make the standards general enough that they could be applied nationally. Part of the basis for that was the fact that the soils and climates in California are so vastly diverse. These specifications, which are very general and open source, are available online for free. You can simply go to urbantree.org. There are four sections: Planting, Soil, Tree Preservation, and Irrigation.

A lot of people have been using the specifications, and I have received a lot of positive feedback. I use the specs myself in my own office. A key factor in the success of these specifications is that we took the soil part of the planting process out of the planting specs and gave it its own section. This is very consistent with the way that the Construction Specifiers Institute wants you to write specs, and it is philosophically consistent with the process. We don’t want the landscape contractors to show up with the tree and a shovel and “fix” the soil while they plant the tree. We want there to first be a soil contractor who works with the soil. The person who plants the tree just shows up, and plants the tree in the previously prepared soil.

Are those specs used outside of the U.S.?

They have been used in Canada. There are some local regulations in the specs, but they were designed to be changeable. We encourage users to make the document local for local needs, plant types, etc.

Let’s move from construction to maintenance. The final principle is “Design for Maintenance.” What do you think is the most important thing to think about when putting together a maintenance plan for urban trees, whether they are street trees or forest patches?

Understand the level of maintenance you can actually provide. I have put together–and I have seen others put together–maintenance plans for all kinds of projects that never get implemented because we make them to be ideal.

Designing for maintenance means you want to design to let the tree grow with the minimal amount of pruning and other maintenance tasks. Tree spacing can make a difference. The wider you space those trees, the more you’ll accomplish that. There is a point of diminishing returns in that equation, but planting oak trees on 15-foot centers is definitely creating maintenance problems that are solved by spreading the trees out further.

Can you give us an example of a situation where you designed for maintenance?

All of my projects are designed for maintenance. I space my trees out a lot further than most designers. I make the space around the tree as large as I can and I make it accessible for maintenance.

Part of the design is getting a tree that has a good root ball. The nursery industry is providing miserable root stock on our trees. I used to think soil was the cause of 90% of all tree problems. I now think soil is 50% of the problem and a big chunk of the remaining 50% is the nursery stock we are putting in the ground –with roots that are too deep in the root ball and circling girdling roots around the trunk of the tree.

If a huge pile of money dropped in your lap with instructions that it had to be used for research related to urban forestry, what would you do?

There is research that could be done related to all ten principles in my book! But I would say we need a lot more research into soil. There are a lot of crazy ideas out there, such as sand-based, structural soil, that have never been researched or for which very little research has been done. We need more research on the use of manufactured soil, and the whole issue of preservation of soil structure during stripping, shipping, handling, and installation. That is an area I think has great promise.

I’m in the process of trying to retire. One of my retirement projects is being on the Board of Trustees for the Tree Fund, and we did have a pile of money magically drop into our hands. We issued about $350,000 in research grants last year, but we had to turn many good proposals away. We are trying to build the endowment. We probably had five good grants for every one that we accepted. There is plenty of research to be done! Recent Tree Fund grants include work on areas such as soils, nursery production, documenting tree benefits, and assessing tree risk so we can safely maintain large trees in our cities.  A complete list of grants can be found at Treefund.org.

You have been involved with urban trees for decades, and you have been responsible for many innovations and standards related to urban tree planting and soil specifications. What is one of the greatest lessons you’ve learned about urban/community forestry?

Be patient. [Tree geneticist] Frank Santamour, a mentor of mine said, “It simply takes ten years to grow a ten-year-old tree, and you don’t learn much about a tree until it is at least ten years old.” In the process of building a house, community, or commercial development, people forget that. You may work with a client who is no longer involved in your project by the time a problem with the trees arises. It is rare that someone comes back to me and says, “Remember that work we did 15 years ago? We have a little problem we need to solve.” All too often, you drive by a project and see that no one is even caring if things worked or didn’t.

Patience is also very important if you are trying to change laws and attitudes. Decision-makers, including landscape architects and engineers, need more education and they need to be inspired to learn about soil and trees.

Based on your experience working with landscape architects and engineers, what is the best way to do that?

One on one, through relationship building. Keep praising the people who are doing a good job and needling the people who are not.

In our last issue, we interviewed Suzanne Simard from the University of British Columbia, who told us about the ways trees use mycorrhizal networks not just to nourish themselves, but to share nutrients and information with nearby trees. To what extent should we be thinking of these networks when planning, designing, and planting urban trees-beyond including mycorrhizae in the soil mix?

That feeds right into my approach: do not think about digging a hole for this tree and making the hole really good; think of having a sheet of interconnected material that covers the site. I often talk about soil volume, but I’d almost rather put a foot of something good down over a large area of something bad, than three feet of something good over a smaller area. In most urban tree plantings, trees don’t interact well because of the soil treatment.

Urban trees can grow just fine without mycorrhizae. Work by Christina Wells of Clemson University showed that urban trees with mycorrhizae and those without grew about the same. This is because in urban areas, we replace all that stuff that the mycorrhizae give the tree with fertilizer and irrigation. We have all of these crutches to compensate for the fact that we don’t have mycorrhizae.

The second part of the problem is that we think we can just put a small amount of mycorrhizal product into the soil mix and that is going to solve the problem. There are thousands of species of mycorrhizae, and when you buy your little bag of mycorrhizae, you get three or four. Most trees have very specific fungi that are suited for that tree. Francesco Ferrini of the University of Florence in Italy has found that if you really want to be successful with mycorrhizae in urban areas, you should use specific species for specific trees.

The problem with mycorrhizae in an urban setting is that you must have the right soil conditions for them to grow. If you simply sprinkle mycorrhizae around a tree where you have highly compacted or high pH soil, they will not do anything. But if you have the right soil conditions-pH, drainage, compaction, and organic matter-the mycorrhizae just start to appear, because their spores are everywhere. That suggests that if you focus on making the space right for the tree, the mycorrhizal problem will solve itself. That’s not to say you can’t improve the mycorrhizal conditions – Ferrini’s work does show that you can.  If I was going to do anything to improve mycorrhizal community when planting a tree, I’d go find that species of tree out in the forest, take a five-gallon bucket, collect some leaf duff around that tree, and spread it around the hole in which I’m planting the tree.

“Make Space for Roots” seems like one of the most challenging principles, given the limited amount of space in urban areas. You helped develop a product called Silva Cell to help address this. Tell us about it. (How does it function in terms of improving life for trees, managing stormwater, distributing loads?)


The big problem in the urban environment is competition for space. First, you have a limited space to plant the tree in. A constant mantra of mine is “Make the hole bigger,” but designers often resist that because they don’t like the way a larger hole looks. I have seen people plant a tree in a hole in the pavement that was smaller than the root ball. They shave the root ball down, that’s how absurd things get.

To make space for roots, you first need to figure out where the roots are going to grow and how to assess the soils when they are under pavement. I devoted a chapter of my book Up By Roots to assessing soils below pavement, but I am still working on this topic and it could really have its own book.

Say I have a sidewalk and the designer says that the largest hole they can put in it is four feet. I’ll ask what that is based on and they’ll say “ADA.” I quickly point out that ADA regulations only require 42 inches and they have five feet of space [for wheelchairs and pedestrians]. But even after pointing out that there is no law requiring them to limit the hole to four feet, they still make the hole too small. So I try to compromise. When you are dealing with a hole that is under eight feet, you really need to fight for space on an inch-by-inch basis. But that only solves the immediate need of the tree. You also need to make space for roots.

One of the answers we came up with to the challenge of making space for roots while still making sure the pavement is supported was Silva Cell. The idea was to create a structure that could support the pavement and be filled with soil. [Silva Cell is a modular suspended pavement system that uses soil volumes to support large tree growth and provide powerful on-site stormwater management through absorption, evapotranspiration, and interception.]

Before, during, and after Silva Cell installation, Toronto

I increasingly tell people to remove the soil from the hole, put the Silva Cells in, and put that same soil back in the hole. The more of that you do, the bigger the tree will become. There is reasonable research to support that. The problem is that we are still competing for space underground with utilities, and we are competing for financial resources on the project budget. Any time you try to put soil under pavement, it is very expensive. Silva Cells are expensive, but they allow you to put unlimited amounts of soil under pavement. DeepRoot Green infrastructure [the company that sells the Silva Cell and other tree care and stormwater management tools] has an analysis piece comparing the benefits of a tree planted in a Silva Cell system vs. a tree without one. If you put a tree in a small hole and you have to replace it every five to 10 years, the tree never moves into the positive range. If you plant it in Silva Cells, the economic benefits pay you back within a few years. Toronto was the first city to embrace Silva Cells, but they are now in many other major cities.

How do Silva Cells perform in terms of retaining and handling stormwater?

Just like any filter, if you make it too small it won’t function well, and if you make water pass through it too fast, it doesn’t do too much to filter out material. It turns out, good old soil is one of the best filters we have. We have had Silva Cells tested for their ability to function as a bioretention facility and they have passed those tests quite well in a number of states and municipalities. Silva Cells afford you the ability to put large areas of soil that can be the filter under the pavement in areas where you don’t have room for large biofiltration beds. It makes a more efficient use of the land.

Is stormwater management part of why cities are using Silva Cell and other below-ground methods?

Yes. In fact, in Toronto, the engineer responsible for stormwater management is one of the biggest advocates of Silva Cells.

Any final words for our readers?

It all starts with the tree. Remember the nursery problem. If you don’t solve that one, the rest of the conversation is moot. Then you have to get that tree into good soil. If you don’t get that one, the conversation is moot. After those two, the other problems get to be relatively easy and cheap to solve.  I have a very positive view of the future, but to get there designers have to stick their hands in the soil and start learning something about it.

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