Tree Damage to Grey Infrastructure: Curbs and Sidewalks
Peter Duinker, Halifax Tree Project
2021-04-26
Anyone paying attention to the street trees in their neighbourhood has witnessed issues where trees have grown large enough to damage either the curb or the sidewalk or both. Because I’m on the lookout for such issues anywhere I travel, I can verify that every city and town I’ve visited (across Canada and Europe as well as a few visits to the USA) has these problems. My photos, starting with the first and going to the last, show what the problems look like. The first is Church St. in Halifax, the second is a street in New Orleans, and the third is a street in Sheffield, UK (you can read a bit about the debacle in Sheffield here). All these three photos show trees just doing what they do best – growing. The bottom right picture shows the kind of damage that trees can inflict on sidewalks and curbs when a storm, like Dorion two autumns ago, blows them over.
We can say with confidence that the best place for a city tree to grow from the point of view of the range of benefits it provides is next to a street. David Foster, graduate student at Dalhousie University, demonstrated this by examining the 22 benefits we posted on this website in relation to a tree’s location (see Foster, 2017). That being the case, why don’t we design our streetscapes to be receptive to trees – and not the little ones we plant, but the big ones they grow into? They start (in the streetscape) as saplings about 3-4 m tall and 4-6 cm in trunk diameter and in a few decades can reach 15 m tall and 40 cm in diameter, and some decades further can reach 25 m tall with trunks 80-100 cm in diameter.
There are basically two choices for avoiding tree damage to curbs and sidewalks. The first is to avoid planting trees in the first place – just pave over the tree lawn, or have just grass, or even plant shrubby trees that don’t grow very big. For people who understand the benefits of trees in the streetscape, that’s a non-starter. At Halifax Tree Project, we heartily and most emphatically disagree with this approach.
The other approach is to design streetscapes to be receptive to growing mature trees. From the perspective of curb and sidewalk damage, two aspects come to mind. The first is the width of the tree lawn, or the distance from curb to sidewalk. In Halifax, including the entire urban core of the Regional Municipality, we have tree lawns ranging in width from about 50 cm all the way to 200 cm and even larger. The young ginkgo growing on Vernon St. enjoys a tree lawn of about 150 cm width, plenty for the tree to grow for a hundred years and jeopardize neither curb nor sidewalk. However, the young trees planted along Caldwell Road in 2013 are in tree lawns only 80 cm wide. This is just asking for trouble in 30-40 years, if indeed the trees can survive that long in this absurd growing space. The message to all the planners and engineers who design streetscapes is clear – make the tree lawn at least 150 cm wide!
The other aspect is related to soil quality, volume, and compaction. The soil environment is exceedingly important for long-term survival, growth, and health of a street tree (actually any tree, for that matter). Trees need lots of underground space to send their feeding and anchoring roots. It’s best to have real soil, not construction rubble/fill or road-construction gravel (as you see in the Caldwell Road photo,below). Road construction people need to build the tree lawn differently than they build the substrate for the road, curb, and sidewalk. Deep, high-quality soil is what trees need.
They also need that soil to be relatively uncompacted. I say “relatively” because, over time, all soil compacts to a certain degree. Tree roots can handle some compaction, but not the amount the subsurface now gets during road construction and re-construction. When the lead waterline to my house on the Halifax peninsula was changed out for copper last June, I monitored the whole process carefully so I would understand better the mechanics of road re-construction. During the backfilling of the hole in the street, I marvelled at how the compactor could turn loose gravel into a surface as hard as concrete. Tree roots would have to work pretty hard to get through that gravel. When my street got its first asphalt covering many decades ago (having been built more than a hundred years ago), I’m sure it was not compacted nearly to the extremes used today. Perhaps good for the roadbed, but lousy for the trees.
Some responsibility for avoiding tree damage to curbs and sidewalks also rests on urban foresters. Perhaps if the tree lawn is like the one on Caldwell Road, no trees should be planted there. If that’s something residents don’t support because they want street trees, those residents need to lobby their municipal councillors to take action on the specifications for street construction and re-construction. Also, the specs for tree planting need to insist that new street trees be planted at least 75 cm from the curb and sidewalk, if possible. Finally, I hypothesize that some street tree species are better than others at working their roots downward into tough growing ground. Species like Norway maple often build a massive root ball that can’t grow downward because of compacted soil and essentially lift the entire tree upward.
In summary, to avoid tree damage to curbs and sidewalks, we need to start with road design and construction and follow that with good planting techniques. This may not eliminate all conflicts between trees on one hand and concrete and asphalt on the other, but surely in this case the former need to trump the latter. Big trees are replaceable only over many decades, but curbs and sidewalks can be replaced in days.