In part one of this post, I gave my recommendations for how to address tree maintenance needs in a smarter and more financially (and ecologically) sustainable way. Those recommendations are great, but also general. I think we benefit the most when we share as much specific information about implementing ideas as possible, so today I want to give examples of cities that are doing things a bit differently in order to improve the health of their urban canopy.
Take inspiration from these cities. They’re doing great stuff.
Minimum Soil Volume Ordinance
Emeryville, CA (San Francisco Bay Area)
Emeryville, California (which receives less than 25 inches, or 63 cm, precipitation annually) is a good example of a city with a soil volume minimum (although there are others; you can read about here). By ordinance, the city requires that all new trees receive the following minimum amount of soil:
- Large species: 1200 cubic feet (34 cubic meters)
- Medium species: 900 cubic feet (25 cubic meters)
- Small species: 600 cubic feet (17 cubic meters)
For more information see Urban, Coder & Kent. You can also read Emeryville’s ordinance here.
“Contract Grown” Trees
New York City, NY
Your city buys many thousands of trees annually, but none of them look as good or perform as well as the tree you picked out at the garden center for your yard and planted yourself. Why?
In my experience, cities often receive the culled, misshapen, and damaged trees, and rarely the premier or fancy grade nursery stock they are paying for. Very few cities enact a rigorous tree selection process because at the time of delivery and in the quantities that cities need, it’s too late. Cities are the single largest consumers of young trees in a region, and they are always in the marketplace for trees. “Contract Grown” trees ensure uniform high-quality trees at prices equal to the open market.
New York City moved to a “Contract Grow” system at the beginning of its million tree program. The initial reason was to secure the 100,000+ diverse trees needed annually to achieve the goals of the program in a short time (less than 8 years) without relying on the vagaries of the marketplace. Even better, the legal framework and contract language was all paid for by New York City, and they are willing to share those documents with other cities wanting to set up their own contract growing programs.
I used the same “Contract Grow” process for large scale wetland restorations in urban locations. These projects required hundreds of thousands of wetland plants to be supplied in a few week time period and to a single location. Contract growing with a few qualified nurseries was the only way this problem could be solved, and turned out to be the best way to ensure high-quality wetland plants.
Your city does not need to be the size of New York to do this; in fact, all cities of more than 5,000 residents should buy the majority of their new trees as “contract grown.” Cities have the bulk buying power to determine tree quality: start using it.
Young tree care
In my early 20’s, after graduating in horticulture from the Irish Botanic Gardens, I went to work at a nursery and landscape firm in Munich. I met and became friends with a colleague, Heinrich. Heinrich was an official “Tree Steward” and our leader – and he had convinced Munich and a few suburbs to pay trained professionals one Deutsche Mark per tree (about $3 to $5 in 2015) to take care of newly planted trees.
This young tree care program all operated on an honor system. It usually involved pruning to remove co-dominant stems, promote strong central leaders, inspecting trunks for injuries, and finding and removing girdling roots and water sprouts of young trees in the public right-of-way. We also added mulch or trunk protection (tree shelters) as needed. These young trees had all been in the ground less than three years and were mainly in boulevard grass strips. Heinrich and I spent many Saturday working on young trees in just this way.
It was the best part-time job I’ve ever had. I believe there are many tree care professionals who would be happy to care for city trees like this for $3 per tree. I don’t know how long the program lasted in Munich or if it was done anywhere else; it was one of the most innovative and efficient ways of growing a healthy urban forest I’ve ever encountered.
Tree-friendly sidewalk repairs
In the last 15 years, two groundbreaking investigative studies came out of Minneapolis regarding high numbers of trees of large trees collapsing catastrophically in moderate wind storms. University of Minnesota Professor Gary Johnson studied these unplanned street tree failures to try to figure out what the typical causes of collapse were.
The tree failures happened in two common ways:
- Tree trunks snapped at the base, while the root plate remained intact in the ground.
- Trees fell over or tipped, pulling the root system out with the sidewalk or curb as they fell.
The first study found that trees planted too deeply 10-20-30 years earlier had an extremely high incidence of snapping at their bases, often with no cavities or rot present in the trunk. These young trees had been planted on average 6” to 12” (15 cm to 30 cm) below their trunk/root collar union, causing them to sprout adventitious roots that formed a second root system above the original one. This second root system was composed of ad-hoc roots sprouting randomly with a huge diversity of sizes, including girdling roots that wrapped the trunk and choked the circulatory transport system of the tree. As they increased in girth, the pressure on the trunk became enormous, such that modest wind storms caused the trees to snap at these pressure points. The girdling roots had changed the mechanics of the tree so that the fulcrum had been moved above the trunk flare, where the greatest mass of wood is located and which is the strongest part of the tree. This all could have been avoided if the trees hadn’t been planted too deep.
The second study showed an exceptionally strong correlation with trees tipping over with their root plate exposed when sidewalk improvements (construction) had occurred around their root systems one to three years earlier. These trees were planted in 5’ to 6’ (1.5 m to 1.8 m) wide concrete sidewalks; whenever the roots heaved the concrete they created trip-and-fall hazards. These concrete panels were replaced by digging up and removing the concrete, cutting all the adjacent tree roots, making a new concrete form, and pouring new panels. This was safer from a pedestrian standpoint, but the sidewalk repair process unintentionally made these trees twice as likely to fail in modest storms. As a result of this finding, Minneapolis Public Works is creating sidewalk and curb repair protocols that do not include cutting existing anchor tree roots.
In both studies, seemingly unimportant decisions for trees led to terrible consequences, costing many millions in damage to people and property citywide. What is the lesson? Simple mistakes like cutting tree roots under sidewalks or planting young trees too deeply results in huge and unexpected operations and maintenance costs. Take the time (and modest amount of money) to plant the tree in a safer manner from the outset.
Maintenance needs to occur at various stages of a tree’s life, starting before it’s even planted and extending into (seemingly) unrelated areas like sidewalk repairs. By making a few key, smart decisions about what trees we plant, how we plant them, and whether we continue to treat them with care when doing work nearby, we can avoid some serious long-term expenses.
By sharing what is being done well in communities like those mentioned above, we can demonstrate that it’s possible to maintain trees adequately and without breaking the bank. Does your city have any best practices around tree care maintenance that you want to share? Email us at [email protected] and we’ll compile them into a future post.
L. Peter MacDonagh is the director of science and design for The Kestrel Design Group.