Ever since humans began farming and building cities 12,000 years ago, there has been an epic struggle between nature and infrastructure. This conflict continues today as we mow our lawn, rake leaves and pull or spray weeds. We love nature in our city, but only when it behaves as we intended. This brings me to my subject: Trees and Sidewalks. One of the most common concerns I receive when planting trees in neighborhoods is, “Isn’t that tree too close to the sidewalk?” We would like to avoid the conflict between trees and human infrastructure, damage does happen. Trees heave sidewalks, they occasionally fall and damage houses, and given the right circumstances, they can clog a sewer line.
I was recently talking to Nancy Buley from J. Frank Schmidt & Son Co., one of the country’s leading suppliers of landscape trees. Nancy talked about tree-years, the conversion of a tree’s life compared to a human life. She suggested the ratio should be about 1 human year to 20 tree years. I like this perspective because it challenges us to think longer-term when planting trees.
Let’s consider expected lifetimes:
[i] Andy Lutz, Andy Lutz, Chief Engineer, Department of Public Works, City of Indianapolis
[ii] World Bank, 2013
[iii] Jerome Delbridge. Approximations based on USDA Forest Service “City Of Indianapolis, Indiana Municipal Forest Resource Analysis.” April 2008; and Arboriculture & Urban Forestry 34(3): May 2008. “Determining the Age of Streetside Trees with Diameter at Breast Height-based Multifactorial Model”
In the life of a street-tree, a tree can expect to see at least two sidewalks poured and outlive the person who planted it. When we think of trees damaging sidewalks, it is helpful to realize that the sidewalk will likely require replacement twice despite any damage inflected by the neighboring tree. Take this, and add in the great benefits trees have on our lives, and I believe the benefits of street trees greatly outweigh the risk of damage to sidewalks.
We need to do what we can to minimize damage to infrastructure while maximizing benefits we gain form trees. To do this, we must realize the Impermanence of sidewalks compared to the permanence of trees.
A large sycamore (Platanus occidentalis) planted in a 2.5’ tree lawn in a Pittsburgh neighborhood.