Thread
For some reason this old tweet just got served up on my for you page, so I thought I'd do a little thread on how planting street trees is more complicated than people think, and why that matters.
First of all, I'm very much pro-street tree. They're great. But there's a tendency to treat trees as a kind of magic pixie dust that you can sprinkle over a city and make things better. Urbanists often describe trees as "cheap," "easy," or "simple" interventions...
...while also describing the benefits of tree planting in quantitative, very human terms, sometimes hyperbolic language. There's a kind of "one weird trick to shed stubborn belly fat" tone to a lot of it.
I think the expectation that you can just stick a little tree in a hole in the sidewalk then sit back and wait for the improved tax revenues, lower crime rates, and better air quality to roll in is unrealistic, and sometimes actively harmful.
This leads to arboriculture programs that aggressively emphasize numbers of trees planted (A thousand trees! A million! A trillion!) rather than creating streets and cities in which trees can thrive and produce really great public spaces.
So a few notes on what it actually takes to create those great 'grammable images: 
First of all, plants need water. The degree to which this is an issue depends on the tree and the climate, but it's rare for street trees to have a permanent irrigation system. However, almost all trees require watering during establishment, or during extreme heat or drought.
The most common way of doing this is with "gator bags," semi porous bags that you can fill with a hose, and which slowly seep water into the soil. They work ok, and especially for new plantings that have shallower root systems, and need frequent watering. 
These also work well from a labor perspective, since you can fill them and then forget about them for a couple days. You can also bring them out if you're having an extreme heat wave to water more established trees, but cities rarely do this in practice.
Sometimes street trees receive permanent drip irrigation, but I actually think this isn't a great practice. If you're planting the right trees for your climate, and planting them in appropriate environments (more on this later) I don't think this should be necessary.
Next, as everyone knows, trees need light. This can sometimes be challenging in areas with lots of tall buildings. East/west streets get more light, generally, and trees on the north side of the street will get more, at least in the northern hemisphere.
Different trees also have different degrees of shade tolerance. If you have a really shady environment, you may need to use a smaller tree (like the carpinus pictured), that in the wild would grow in the shady understory of larger trees. 
Conversely, sometimes trees can get too much light in urban environments. Glass curtain walls can sometimes reflect a lot of glare, overheating or singing street trees, although that's rarer.
The other thing trees need is oxygen. Most of us get taught that animals breathe oxygen and produce co2, then plants take up co2 and produce oxygen. That’s true, but plants need oxygen too. And notably, they need it underground.
This is important, because urban soils tend to be very heavily compacted. Sometimes this is just the product of feet and heavy vehicles pounding the ground over and over, but we also deliberately compact the soil under roads and sidewalks to provide a stable base for pavement.
This hurts trees two ways. First, compacted soils have less open pore space to hold both oxygen and water, depriving roots of those resources. Secondly, when soils are heavily compacted, tree roots sometimes literally can't push their way into the surrounding ground.
This is what causes so many street trees to fail. People cut a tiny hole into the pavement and plop a tree into it with high hopes. But the surrounding soil is basically hard as a rock. The roots have maybe a cubic yard of space to work with, and that's it.
Trees develop girdling roots as their roots circle around in their little hole, looking for somewhere to go. They develop aerial roots, looking for oxygen closer to the air. Or they run underneath sidewalks, looking for a little more space. 
This is the biggest reason why I think we need to be more realistic about the challenges of urban tree planting. More and more cities are embracing a model where homeowners request a tree in the sidewalk in front of their house, in exchange for taking care of it initially.
This can be tricky because you are relying on individual, non-experts to take care of your urban canopy, but these initiatives also generally also produce the tiniest, most bare-bones tree pits imaginable. Look at this guy!
Public participation, non-profits, public-private partnerships can all play a role in urban forestry, but the important thing to understand is that creating a great urban canopy is an *infrastructural* problem.
So how do you grow great urban trees? First, give them enough space. A lot of tree plantings try to get away with a cubic yard of soil in a tree pit with a 3'x3' opening. I would triple that: something more like 4'x6'.
This can be tricky with narrow sidewalks and utility conflicts, but there are a few tricks to be more efficient with limited space. First of all: combine planting areas. Three trees sharing 6 yards of soil are going to be happier than three trees each with their own 2 yards. 
The second trick is to allow trees to use soils outside of their pits, especially soil under pavement. There are basically two major ways of doing this: suspended pavement systems and structural soils.
The best-known suspended pavement system are Silva Cells: these are basically grids of plastic material that are strong enough to support sidewalks or pavement, but can be filled with higher-quality, uncompacted planting soils to allow roots to spread out. 
Structural soils work on a similar concept. These are custom-mixed engineered soils that are able to be compacted enough to support pavement, while also have enough pore space for root growth, oxygen, etc.
The best-known example is CU soil, invented at Cornell. Its made of very coarse crushed stone, mixed with some clay and a "tackifier" that retains water and moisture. The angular crushed stone locks together to create a stable base, but with lots of open space for roots. 
There are also "sand-based" structural soils, that work on a similar concept, but using angular sand. The pores are smaller, but it works more or less the same way. These are what I use more frequently, mostly for logistical reasons.
These systems make a huge difference but it's important to understand how they work. You don't open up a hole in an existing sidewalk, put a tree in and backfill with structural soils. You have to use them in the area *around* the tree pit, when you're installing the pavement.
This gets to my bigger point: the way you get a great urban canopy isn't by planting trees. It's by creating great spaces for trees to grow. And that's something that has to be done at a infrastructural level. Most of the time you cant just plug a tree in to what already exists.
Because of this, I'd love to see cities shift from saying "we're going to plant this many trees" to something like "we're going to redesign this many blocks to support great urban canopy." Ultimately, that's the approach thats going to create the outcomes we're hoping for.
